x86: Stop clearing RAX for BIST in initCPU.

x86: Stop clearing RAX for BIST in initCPU.

This doesn't actually change any behavior since RAX was being zeroed
anyway, but since we don't and almost certainly never will have a BIST
and the BIST is optional even in real hardware, we can drop it and
simplify initCPU a little further.

This reduces x86's initCPU function to just an invocation of
InitInterrupt's invoke.

Change-Id: I56b1aae2c1a738ef7ffabcf648dd7d0fb819d4e0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24187
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>