gas: ensure sections contents is zero for BFD_RELOC_PPC*_TLSM on XCOFF.

gas: ensure sections contents is zero for BFD_RELOC_PPC*_TLSM on XCOFF.

AIX ld expects the section contents for relocations BFD_RELOC_PPC_TLSM
or BFD_RELOC_PPC64_TLSM to be zero.
Actually, it seems to be the case for all the TLS relocations
generated by AIX assembly but only these two are mandatory.

	* config/tc-ppc.c (md_apply_fix): Adjust addend to nullify
	section contents for BFD_RELOC_PPC_TLSM or
	BFD_RELOC_PPC64_TLSM.
	* testsuite/gas/ppc/xcoff-tlsm-32.d: New test.
	* testsuite/gas/ppc/xcoff-tlsm-64.d: New test.
	* testsuite/gas/ppc/xcoff-tlsm.s: New test.
	* testsuite/gas/ppc/aix.exp: Run new tests.