Binutils 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.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
