xtensa: fix signedness of gas relocations

xtensa: fix signedness of gas relocations

Change 1058c7532d0b "Use signed data type for R_XTENSA_DIFF* relocation
offsets." changed signedness of BFD_RELOC_XTENSA_DIFF* relocations
substituted for BFD_RELOC_*. This made it impossible to encode arbitrary
8-, 16- and 32-bit values, which broke e.g. debug info encoding by .loc
directive. Revert this part and add test.

gas/
2016-02-03  Max Filippov  <jcmvbkbc@gmail.com>
	* config/tc-xtensa.c (md_apply_fix): Mark BFD_RELOC_XTENSA_DIFF*
	substitutions for BFD_RELOC_* as unsigned.
	* gas/testsuite/gas/xtensa/all.exp: Add loc to list of xtensa
	tests.
	* gas/testsuite/gas/xtensa/loc.d: New file: loc test result
	patterns.
	* gas/testsuite/gas/xtensa/loc.s: New file: loc test.