don't pass -fstack-clash-protection to mingw when building onetbb

GCC 10.4 aborts compiling oneTBB's task_dispatcher.h with the flag on:

  internal compiler error: in seh_emit_stackalloc, at config/i386/winnt.c:1056
    370 |                 do_throw_noexcept([] { throw; });

a known GCC defect on mingw's SEH targets. It is a hardening flag rather
than a correctness one, so leave it off there.

Also fold the earlier /dev/null fix into a single patches/mingw_gnu_cmake.diff,
as both make oneTBB's GNU.cmake usable for a native mingw build.

Author: kevinushey