RISC-V: Support assembler modifier %got_pcrel_hi.

RISC-V: Support assembler modifier %got_pcrel_hi.

	gas/
	* config/tc-riscv.c: Support the modifier %got_pcrel_hi.
	* doc/c-riscv.texi: Add documentation.
	* testsuite/gas/riscv/no-relax-reloc.d: Add test case for the new
	modifier %got_pcrel_hi.
	* testsuite/gas/riscv/no-relax-reloc.s: Likewise.
	* testsuite/gas/riscv/relax-reloc.d: Likewise.
	* testsuite/gas/riscv/relax-reloc.s: Likewise.