[sanitizer][RISCV] Implement SignalContext::GetWriteFlag for RISC-V

This patch follows the approach also used for MIPS, where we decode the
offending instruction to determine if the fault was caused by a read or
write operation, as that seems to be the only relevant information we have
in the signal context structure to determine that.

Differential Revision: https://reviews.llvm.org/D75168