riscv: Use memcpy to handle unaligned access when fixing R_RISCV_RELATIVE

Although RISC-V Linux will enable the unaligned memory access handler by
default, that is quite expensive in general, using memcpy will be much cheaper
- just break down that into several load/store byte instructions.

ARM and MIPS has similar issue:

ARM: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51456
MIPS: https://gcc.gnu.org/legacy-ml/gcc-help/2005-07/msg00325.html



Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>