Make it explicit we are currently using a signed char

[ktf]

On x86 Linux char is assumed to be signed, however that's an implementation detail.

This makes sure we are explicit about the sign-ness.

[github-actions]

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[ktf]

@davidrohr this fixes one of the two tests on ARM.

[davidrohr]

makes sense, but the error during the RTC test is genuine :(.

[ktf]

Added the specialization for the printout.

[alibuild]

Error while checking build/O2/fullCI for c731b2a at 2024-08-13 16:18:

## sw/BUILD/O2-latest/log
c++: error: unrecognized command-line option '--rtlib=compiler-rt'
c++: error: unrecognized command-line option '--rtlib=compiler-rt'


## sw/BUILD/o2checkcode-latest/log
--
========== List of errors found ==========
++ GRERR=0
++ grep -v clang-diagnostic-error error-log.txt
++ grep ' error:'
/sw/SOURCES/O2/13375-slc8_x86-64/0/GPU/GPUTracking/Global/GPUChainITS.cxx:40:14: error: use '= default' to define a trivial destructor [modernize-use-equals-default]
/sw/SOURCES/O2/13375-slc8_x86-64/0/GPU/GPUTracking/Interface/GPUO2Interface.cxx:49:17: error: use '= default' to define a trivial default constructor [modernize-use-equals-default]
++ [[ 0 == 0 ]]
++ exit 1
--

Full log here.

[ktf]

Verified to fix one of the two ARM tests.