[AIX][Debug] generate an error instead of crash in backend for -gdwarf-5

Before this change -gdwarf-5 on AIX will cause backend crash, because
some DWARF5 sections are not defined in XCOFF.
Explicitly statement -gdwarf-5 as unsupported in frontend on AIX.

GitOrigin-RevId: b2323f43e3cdb52b4e15a7d4f434cd5c64740dd4
2 files changed