RISC-V: Enable x0 base relaxation for relax_pc even if --no-relax-gp.

RISC-V: Enable x0 base relaxation for relax_pc even if --no-relax-gp.

Let --no-relax-gp only disable the gp relaxation for lui and pcrel
relaxations, since x0 base and gp relaxations are different optimizations
in fact, but just use the same function to handle.

bfd/
	* elfnn-riscv.c (_bfd_riscv_relax_pc): Like _bfd_riscv_relax_lui,
	set gp to zero when --no-relax-gp, then we should still keep the
	x0 base relaxation.
	(_bfd_riscv_relax_section): Enable _bfd_riscv_relax_pc when
	--no-relax-gp, we will disable the gp relaxation in the
	_bfd_riscv_relax_pc.