arch-riscv: Implement MHARTID CSR

arch-riscv: Implement MHARTID CSR

This patch implements the MHARTID CSR by intercepting attempts to access
it, similar to the way accesses to the performance counters are
intercepted, to return the thread's context ID.

Change-Id: Ie14a31036fbe0e49fb3347ac0c3c508d9427a10d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/16988
Reviewed-by: Alec Roelke <alec.roelke@gmail.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Alec Roelke <alec.roelke@gmail.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>