gdb/record: Support wfi, sfence.vma, sret and mret instructions in risc-v

During testing of bare-metal applications on QEMU for RISC-V, it was discovered
that the instructions wfi, sfence.vma, sret, and mret were not supported. This
patch introduces support for these instructions. Additionally, it wraps
fetch_instruction function in a try-catch block to gracefully handle errors
that may occur when attempting to read invalid address.

Reviewed-By: Guinevere Larsen <guinevere@redhat.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>