x86: Use the common pseudoInst dispatch function.

x86: Use the common pseudoInst dispatch function.

Instead of hand invoking each individual pseudo inst. New instructions
added in the future will automatically become available without a lot of
extra hand implementation. It also simplifies the x86 ISA description.

Change-Id: Ibb671dc2656e61679b7ed016c51a6c879e12910a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27789
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>