tests: debuginfo: Work around or disable broken tests on powerpc

f16 support for PowerPC has issues in LLVM, therefore we need a small
workaround to prevent LLVM from emitting symbols that don't exist for
PowerPC yet.

It also appears that unused by-value non-immedate issue with gdb applies
to PowerPC targets as well, though I've only tested 64-bit Linux targets.

Signed-off-by: Jens Reidel <adrian@travitia.xyz>