RISC-V: Make R_RISCV_SUB6 conforms to riscv ABI standard

According to the riscv psabi, R_RISCV_SUB6 only allows 6 least significant
bits are valid, but since binutils implementation, we usually get 8 bits
field for it.  That means, the high 2 bits could be other field and have
different purpose.  Therefore, we should filter the 8 bits to 6 bits before
calculate, and then only encode the valid 6 bits back.  By the way, we also
need the out-of-range check for R_RISCV_SUB6, and the overflow checks for
all R_RISCV_ADD/SUB/SET relocations, but we can add them in the future patches.

Passing riscv-gnu-toolchain regressions.

bfd/ChangeLog:

        * elfnn-riscv.c (riscv_elf_relocate_section): Take the R_RISCV_SUB6
	lower 6 bits as the significant bit.
        * elfxx-riscv.c (riscv_elf_add_sub_reloc): Likewise.