x86: Let the pseudoInst dispatch function handle the return value.

x86: Let the pseudoInst dispatch function handle the return value.

When the result is returned to the caller from the pseudoInst dispatch
function, the default behavior is to not store that value using the
guestABI mechanism. In the x86 definition, I accidentally used this
version but then didn't store the result manually. The fix should simply
be to not return the result to the instruction definition and to let the
guestABI mechanism handle everything normally.

Change-Id: Ib69f266ad6314032622e5d8d69e9ff114c62657a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38195
Reviewed-by: Daniel Gerzhoy <daniel.gerzhoy@gmail.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>