x86: Move miscreg initialization to the ISA class.

x86: Move miscreg initialization to the ISA class.

The initCPU function was setting a lot of values to zero or other
initial values, but that's something the ISA object can do as part of
its clear() method. This gets rid of a lot of code that was
individually zeroing registers, and also centralizes responsibility
for those registers in the ISA.

Change-Id: Iafcffd3f9329c39f77009b38b1696f91c36c117e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24185
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>