bfd: RISC-V: relax to gp in more cases.

bfd: RISC-V: relax to gp in more cases.

bfd/ChangeLog:

2017-02-24  Andrew Waterman  <andrew@sifive.com>

	* elfnn-riscv.c (GP_NAME): New macro.
	(riscv_global_pointer_value): Use it.
	(_bfd_riscv_relax_lui): If symbol and global pointer are in same
	output section, consider only that section's alignment.