[JITLink][RISCV] Ignore R_RISCV_RELAX and check R_RISCV_ALIGN

It is fine to not implement and ignore linker relaxation for now, but
we need to check the alignment. Luckily, an alignment of only 2 bytes
is the most common case when interpreting C++ code in clang-repl, and
already guaranteed by the length of compressed instructions.

Differential Revision: https://reviews.llvm.org/D129159