arch-riscv: Add support for fault handling

arch-riscv: Add support for fault handling

This patch adds support for handling RISC-V faults, including tracking
current and previous execution privilege, correctly switching to
the privilege mode specified by CSRs, and setting/storing the PC.  It
also includes changes introduced by patch #9821, which disables
interrupts during handling of a fault.

Change-Id: Ie9c0f29719620c20783540d3bdb2db44f6114fc9
Reviewed-on: https://gem5-review.googlesource.com/9161
Maintainer: Alec Roelke <ar4jc@virginia.edu>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>