RISC-V: Fix BFD_RELOC_RISCV_PCREL_LO12_S patch issue

In commit dff565fcca8137954d6ad571ef39f6aec5c0429c, the fixups
for PCREL_LO12_I and PCREL_LO12_S were mixed, so the "IMM"
field were applied to incorrect position, this caused incorrect
src registers to be encoded.

gas/
	* config/tc-riscv.c (md_apply_fix): Fix PCREL_LO12_S issue.
	* testsuite/gas/riscv/ixup-local.s: Updated for PCREL_LO12_S cases.
	* testsuite/gas/riscv/fixup-local-relax.d: Likewise.
	* testsuite/gas/riscv/fixup-local-norelax.d: Likewise.

Signed-off-by: Jianwei Sun <sunny.sun@corelabtech.com>