RISC-V/GAS: Support more relocs against constant addresses

RISC-V/GAS: Support more relocs against constant addresses

Previously, some pseudoinstructions like "call" only accepted
symbolic addresses and rejected constant addresses with an
esoteric internal error.  This patch enables them by deferring
application of constant relocations to md_apply_fix, rather than
eagerly applying them during instruction assembly.

gas/ChangeLog

2017-01-09  Andrew Waterman <andrew@sifive.com>

	* config/tc-riscv.c (append_insn): Don't eagerly apply relocations
	against constants.
	(md_apply_fix): Mark relocations against constants as "done."