xtensa: fix PR ld/25861

xtensa: fix PR ld/25861

Introduce new relaxations XTENSA_PDIFF{8,16,32} for positive differences
(subtracted symbol precedes diminished symbol) and XTENSA_NDIFF{8,16,32}
for negative differences (subtracted symbol follows diminished symbol).
Don't generate XTENSA_DIFF relocations in the assembler, generate
XTENSA_PDIFF or XTENSA_NDIFF based on relative symbol position.

Handle XTENSA_DIFF in BFD for compatibility with old object files.
Handle XTENSA_PDIFF and XTENSA_NDIFF in BFD, treating difference value
as unsigned.

2020-04-22  Max Filippov  <jcmvbkbc@gmail.com>
bfd/
	* bfd-in2.h: Regenerated.
	* elf32-xtensa.c (elf_howto_table): New entries for
	R_XTENSA_PDIFF{8,16,32} and R_XTENSA_NDIFF{8,16,32}.
	(elf_xtensa_reloc_type_lookup, elf_xtensa_do_reloc)
	(relax_section): Add cases for R_XTENSA_PDIFF{8,16,32} and
	R_XTENSA_NDIFF{8,16,32}.
	* libbfd.h (bfd_reloc_code_real_names): Add names for
	BFD_RELOC_XTENSA_PDIFF{8,16,32} and
	BFD_RELOC_XTENSA_NDIFF{8,16,32}.
	* reloc.c: Add documentation for BFD_RELOC_XTENSA_PDIFF{8,16,32}
	and BFD_RELOC_XTENSA_NDIFF{8,16,32}.

binutils/
	* readelf.c (is_none_reloc): Recognize
	BFD_RELOC_XTENSA_PDIFF{8,16,32} and
	BFD_RELOC_XTENSA_NDIFF{8,16,32}.

gas/
	* config/tc-xtensa.c (md_apply_fix): Replace
	BFD_RELOC_XTENSA_DIFF{8,16,32} generation with
	BFD_RELOC_XTENSA_PDIFF{8,16,32} and
	BFD_RELOC_XTENSA_NDIFF{8,16,32} generation.
	* testsuite/gas/xtensa/loc.d: Replace BFD_RELOC_XTENSA_DIFF16
	with BFD_RELOC_XTENSA_PDIFF16 in the expected output.

include/
	* elf/xtensa.h (elf_xtensa_reloc_type): New entries for
	R_XTENSA_PDIFF{8,16,32} and R_XTENSA_NDIFF{8,16,32}.

ld/
	* testsuite/ld-xtensa/relax-loc.d: New test definition.
	* testsuite/ld-xtensa/relax-loc.s: New test source.
	* testsuite/ld-xtensa/xtensa.exp (relax-loc): New test.