arch-gcn3, gpu-compute: Fix issue when reading const operands

arch-gcn3, gpu-compute: Fix issue when reading const operands

Currently, when an instruction has an operand that reads a const
value, it goes thru the same readMiscReg() api call as other
misc registers (real HW registers, not constant values). There
is an issue, however, when casting from the const values (which are
32b) to higher precision values, like 64b.

This change creates a separate, templated function call to the GPU's
ISA state that will return the correct type.

Change-Id: I41965ebeeed20bb70e919fce5ad94d957b3af802
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29927
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>