fix the RcppParallel.dll half of the downstream import check

Two reasons it was never actually running, both visible in the CI log
even though the job passed:

- the path was built with paste0("libs", .Platform$r_arch), giving
  'libsx64' rather than 'libs/x64'. system.file() returned "" for that,
  so objdump was handed '/RcppParallel.dll' and reported "No such file".
  Use archSystemFile(), which the package already has for this, and fail
  loudly if the library still can't be found.

- on aarch64 the runner has an x86_64 objdump from C:/mingw64 ahead of
  Rtools' own on the PATH, and it cannot read an aarch64 PE, so both
  halves of the check skipped there. Try the objdump sitting beside the
  compiler first, and fall back through the remaining candidates.

Author: kevinushey

.github/scripts/tbb-downstream-check.R

@@ -104,38 +104,59 @@ dllName <- paste0("check", .Platform$dynlib.ext)
 # too rather than carrying a copy of its own. Two runtimes would be a silent
 # failure -- an observer registered with one would never fire for arenas owned
 # by the other -- so it is worth asserting rather than assuming.
-tbbImports <- function(dll) {
 
-   objdump <- Sys.which("objdump")
-   if (!nzchar(objdump)) {
-      writeLines("** objdump not found; skipping the import table check")
-      return(NULL)
-   }
+# objdump has to match the target architecture, and the first one on the PATH
+# may well not: the aarch64 runner has an x86_64 objdump from C:/mingw64 ahead
+# of Rtools' own, and it exits non-zero having read nothing. Look beside the
+# compilers first, since those are necessarily right for the target
+objdumpCandidates <- function() {
 
-   output <- suppressWarnings(
-      system2(objdump, c("-p", shQuote(dll)), stdout = TRUE, stderr = TRUE)
-   )
-   status <- attr(output, "status")
+   compilers <- Sys.which(c("gcc", "clang", "cc"))
+   beside <- file.path(dirname(compilers[nzchar(compilers)]), "objdump.exe")
+
+   candidates <- unique(c(beside, Sys.which("objdump")))
+   candidates[nzchar(candidates) & file.exists(candidates)]
+
+}
 
-   # keep only the import tables. the export table follows them, and lists the
-   # library's own inlined TBB instantiations -- left in, it would be absorbed
-   # into the last 'DLL Name:' block and credit that library with TBB symbols
-   # it never imported
-   exports <- grep("export table|The Export Tables", output)
-   if (length(exports))
-      output <- output[seq_len(exports[[1L]] - 1L)]
+tbbImports <- function(dll) {
 
    # each imported library opens a 'DLL Name:' block listing the symbols taken
-   # from it, and runs until the next such block
-   starts <- grep("DLL Name:", output, fixed = TRUE)
-
-   # don't let an objdump that couldn't read the file pass as 'no imports': the
-   # runner's objdump may be built for another target (an x86_64 one cannot
-   # read an aarch64 PE, and exits non-zero having printed nothing)
-   if (!length(starts) || (is.numeric(status) && status != 0L)) {
-      fmt <- "** '%s' could not read the import table of '%s'; skipping the check"
+   # from it; finding at least one is how we know objdump really read the file,
+   # rather than failing in a way that would look like 'imports nothing'
+   starts <- integer()
+   output <- character()
+
+   for (objdump in objdumpCandidates()) {
+
+      output <- suppressWarnings(
+         system2(objdump, c("-p", shQuote(dll)), stdout = TRUE, stderr = TRUE)
+      )
+      status <- attr(output, "status")
+
+      # keep only the import tables. the export table follows them, and lists
+      # the library's own inlined TBB instantiations -- left in, it would be
+      # absorbed into the last 'DLL Name:' block and credit that library with
+      # TBB symbols it never imported
+      exports <- grep("export table|The Export Tables", output)
+      if (length(exports))
+         output <- output[seq_len(exports[[1L]] - 1L)]
+
+      starts <- grep("DLL Name:", output, fixed = TRUE)
+      if (length(starts) && !(is.numeric(status) && status != 0L))
+         break
+
+      fmt <- "** '%s' could not read the import table of '%s'"
       writeLines(sprintf(fmt, objdump, basename(dll)))
       writeLines(output)
+
+      starts <- integer()
+
+   }
+
+   if (!length(starts)) {
+      fmt <- "** no usable objdump for '%s'; skipping the import table check"
+      writeLines(sprintf(fmt, basename(dll)))
       return(NULL)
    }
 
@@ -180,10 +201,10 @@ if (.Platform$OS.type == "windows") {
    }
 
    # and RcppParallel itself must be a client of that same runtime
-   rcppParallelDll <- file.path(
-      system.file(paste0("libs", .Platform$r_arch), package = "RcppParallel"),
-      "RcppParallel.dll"
-   )
+   rcppParallelDll <- RcppParallel:::archSystemFile("libs", "RcppParallel.dll")
+   if (!file.exists(rcppParallelDll))
+      stop("could not locate RcppParallel.dll within the installed package")
+
    providers <- tbbImports(rcppParallelDll)
    if (!is.null(providers) && !identical(providers, "tbb.dll"))
       stop("RcppParallel.dll does not take its tbb symbols from 'tbb.dll' ",