gas: enable PC-relative diff relocations on sparc64

gas: enable PC-relative diff relocations on sparc64

gas/
	* config/tc-sparc.c (tc_gen_reloc): Convert BFD_RELOC_8/16/32/64
	into the corresponding BFD_RELOC_8/16/32/64_PCREL relocation
	when requested.
	* config/tc-sparc.h (DIFF_EXPR_OK): Define to enable PC-relative
	diff relocations.
	(TC_FORCE_RELOCATION_SUB_LOCAL): Define to ensure only supported
	relocations are made PC-relative.
	(CFI_DIFF_EXPR_OK): Define to 0 to force BFD_RELOC_32_PCREL to
	be used directly, since otherwise BFD_RELOC_SPARC_UA32 will be
	used for .eh_frame which cannot in general be converted to a
	BFD_RELOC_32_PCREL due to alignment requirements.