gas: default TC_VALIDATE_FIX_SUB to 0

gas/write.c provides a fallback TC_VALIDATE_FIX_SUB define that can be
a problem for some targets, the problem being that a non-zero
definition of TC_VALIDATE_FIX_SUB says that some uses of fx_subsy are
OK, in effect that the target will handle fx_subsy in md_apply_fix
and/or tc_gen_reloc.  A lot of targets don't have the necessary
md_apply_fix and tc_gen_reloc support.  So a safer default is to
disallow fx_subsy by default.

I've had a good look over target usage of fx_subsy, and think I've
caught all the cases where targets need TC_VALIDATE_FIX_SUB.  Possible
failures would be limited to alpha, microblaze, ppc and s390 (the
targets that define UNDEFINED_DIFFERENCE_OK), or targets that generate
fixups with BFD_RELOC_GPREL32/16 and use a syntax explicitly showing
a difference expression.

	* write.c (TC_VALIDATE_FIX_SUB): Default to 0.
	* config/tc-hppa.h (TC_VALIDATE_FIX_SUB): Define.
	* config/tc-microblaze.h (TC_VALIDATE_FIX_SUB): Define.
	* config/tc-alpha.h (TC_VALIDATE_FIX_SUB): Define for ECOFF.
	* config/tc-ppc.h (TC_VALIDATE_FIX_SUB): Don't define for ELF.
	Do define for XCOFF.

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