gpu: Use X86ISA instead of TheISA in src/gpu-compute.

gpu: Use X86ISA instead of TheISA in src/gpu-compute.

These files are nominally not tied to the X86ISA, but in reality they
are because they reach into the GPU TLB, which is defined unchangeably in
the X86ISA namespaces, and uses data structures within it. Rather than try
to pretend that these structures are generic, we'll instead just use X86ISA
instead of TheISA. If this really does become generic in the future, a
base class with the ISA agnostic essentials defined in it can be used
instead, and the ISA specific TLBs can defined their own derived class
which has whatever else they need. Really the compute unit shouldn't be
communicating with the TLB using sender state since those are supposed
to be little notes for the sender to keep with a transaction, not for
communicating between entities across a port.

Change-Id: Ie6573396f6c77a9a02194f5f4595eefa45d6d66b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34174
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>