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.