RISC-V: Support subtraction of .uleb128.

https://github.com/riscv-non-isa/riscv-elf-psabi-doc/commit/96d6e190e9fc04a8517f9ff7fb9aed3e9876cbd6

There are some known limitations for now,

* Do not shrink the length of the uleb128 value, even if the value is reduced
after relaxations.  Also reports error if the length grows up.

* The R_RISCV_SET_ULEB128 needs to be paired with and be placed before the
R_RISCV_SUB_ULEB128.

bfd/
	* bfd-in2.h: Regenerated.
	* elfnn-riscv.c (perform_relocation): Perform R_RISCV_SUB_ULEB128 and
	R_RISCV_SET_ULEB128 relocations.  Do not shrink the length of the
	uleb128 value, and report error if the length grows up.  Called the
	generic functions, _bfd_read_unsigned_leb128 and _bfd_write_unsigned_leb128,
	to encode the uleb128 into the section contents.
	(riscv_elf_relocate_section): Make sure that the R_RISCV_SET_ULEB128
	must be paired with and be placed before the R_RISCV_SUB_ULEB128.
	* elfxx-riscv.c (howto_table): Added R_RISCV_SUB_ULEB128 and
	R_RISCV_SET_ULEB128.
	(riscv_reloc_map): Likewise.
	(riscv_elf_ignore_reloc): New function.
	* libbfd.h: Regenerated.
	* reloc.c (BFD_RELOC_RISCV_SET_ULEB128, BFD_RELOC_RISCV_SUB_ULEB128):
	New relocations to support .uleb128 subtraction.
gas/
	* config/tc-riscv.c (md_apply_fix): Added BFD_RELOC_RISCV_SET_ULEB128
	and BFD_RELOC_RISCV_SUB_ULEB128.
	(s_riscv_leb128): Updated to allow uleb128 subtraction.
	(riscv_insert_uleb128_fixes): New function, scan uleb128 subtraction
	expressions and insert fixups for them.
	(riscv_md_finish): Called riscv_insert_uleb128_fixes for all sections.
include/
	* elf/riscv.h ((R_RISCV_SET_ULEB128, (R_RISCV_SUB_ULEB128): Defined.
ld/
	* testsuite/ld-riscv-elf/ld-riscv-elf.exp: Updated.
	* testsuite/ld-riscv-elf/uleb128*: New testcase for uleb128 subtraction.
binutils/
	* testsuite/binutils-all/nm.exp: Updated since RISCV supports .uleb128.