Rework the R_NEG support on both gas and ld for the PowerPC AIX targets, in order...

Rework the R_NEG support on both gas and ld for the PowerPC AIX targets, in order to manage C++ exceptions built with GCC.

bfd	PR binutils/21700
	* reloc.c (BFD_RELOC_PPC_NEG): New relocation.
	* bfd-in2.h: Regenerate.
	* libbfd.h: Regenerate.
	* coff-rs6000.c (_bfd_xcoff_reloc_type_lookup): Add
	BFD_RELOC_PPC_NEG handler.
	(xcoff_reloc_type_neg): Correctly substract addend.
	* coff64-rs6000.c (xcoff64_howto_table): Add R_NEG_32
	howto.
	(xcoff64_rtype2howto): Add handler for R_NEG_32.
	(xcoff64_reloc_type_lookup): Add BFD_RELOC_PPC_NEG handler.
	* xcofflink.c (xcoff_need_ldrel_p): Check output section
	for R_POS-like relocations. New argument added.
	(xcoff_mark): Adapt to new xcoff_need_ldrel_p argument.
	(xcoff_link_input_bfd): Likewise.

gas	* config/tc-ppc.c (ppc_get_csect_to_adjust): New function.
	(ppc_fix_adjustable): Manage fx_subsy part.
	(tc_gen_reloc): Create second relocation when both
	fx_addsy and fx_subsy are provided.
	* config/tc-ppc.h (RELOC_EXPANSION_POSSIBLE): New define.
	(MAX_RELOC_EXPANSION): Likewise.
	(TC_FORCE_RELOCATION_SUB_SAME): Likewise
	(UNDEFINED_DIFFERENCE_OK): Likewise
	* testsuite/gas/all/gas.exp: Skip difference between two
	undefined symbols test.

ld	* testsuite/ld-powerpc/aix52.exp: Add new test.
	* testsuite/ld-powerpc/aix-neg-reloc-32.d: New test.
	* testsuite/ld-powerpc/aix-neg-reloc-64.d: New test.
	* testsuite/ld-powerpc/aix-neg-reloc.ex: New test.
	* testsuite/ld-powerpc/aix-neg-reloc.s: New test.