arch-gcn3: implement instruction s_setreg_b32

arch-gcn3: implement instruction s_setreg_b32

Instruction s_setreg_b32 was unimplemented, but is used by hipified
rodinia 'srad'. The instruction sets values of hardware internal
registers. If the instruction is writing into MODE to control
single-precision FP round and denorm modes, a simple warn will be
printed; for all other cases (non-MODE hw register or other
precisions), panic will happen.

Change-Id: Idb1cd5f60548a146bc980f1a27faff30259e74ce
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29949
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Xianwei Zhang <xianwei.zhang@amd.com>