[GSYM] Update gSym unit test with stable / portable path (#131204)

This patch adds `-fdebug-compilation-dir=.` to the clang invocation in
`llvm/test/tools/llvm-gsymutil/ARM_AArch64/macho-gsym-merged-callsites-dsym.yaml`
and updates the rest of the test accordingly.

Previously, without the newly added argument, the test would
significantly change every time it was regenerated due to the generation
script using a temporary directory as the compilation directory. See
discussion in #129562 for
additional context.

This patch ensures determinism across test update runs - if the test
hasn't been updated and we try to regenerate the contents, nothing will
change.

Author: alx32