[RISCV] Support -m[no-]strict-align options

To match GCC.

Options `-m[no-]strict-align` are aliases of `-m[no-]unaligned-access`
in clang, but there is no corresponding option in GCC.

Support of `-m[no-]unaligned-access` in GCC may be needed to align
Clang/GCC.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D155456