arch-gcn3: Remove invalid assert when reading EXEC_LO

arch-gcn3: Remove invalid assert when reading EXEC_LO

This assert assumed all reads to EXEC_LO would be
64b, that is, we would always read the entire EXEC
mask. This is invalid as some kernels read only
the low 32b of EXEC.

The write to EXEC_LO is also updated to handle 32b
writes.

Change-Id: Ifeb167578515bf112b1eab70bbf2201a5e936358
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29960
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>