xtensa: gas: don't keep relocations for constants

xtensa: gas: don't keep relocations for constants

xtensa gas chokes on 8/16 bit data entries representing constant symbols
because it leaves BFD_RELOC_8/BFD_RELOC_16 fixups for which xtensa BFD
cannot emit relocations. Resolve fixups for constant symbols in
md_apply_fix.

gas/
2019-01-28  Max Filippov  <jcmvbkbc@gmail.com>

	* config/tc-xtensa.c (md_apply_fix): Mark fixups for constant
	symbols as done in md_apply_fix.
	* testsuite/gas/all/forward.d: Don't XFAIL for xtensa.