arch-x86: Make JRCXZ instruction do 64-bit jump

arch-x86: Make JRCXZ instruction do 64-bit jump

Per the AMD64 Architecture Programming Manual:

The size of the count register (CX, ECX, or RCX) depends on the
address-size attribute of the JrCXZ instruction. Therefore, JRCXZ can
only be executed in 64-bit mode

and

In 64-bit mode, the operand size defaults to 64 bits. The processor
sign-extends the 8-bit displacement value to 64 bits before adding it
to the RIP.

This patch also renames the instruction from JRCX to JRCXZ to match the
language in the programming manual.

Change-Id: Id55147d0602ff41ad6aaef483bef722ff56cae62
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40195
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>