Instructions LDARB, LDARH, LDAXPW, LDAXP, STLRB, STLRH, STLXP, STLXPW, STXP,
STXPW have been added before, but they are not enabled. This CL enabled them.

Change the form of LDXP and LDXPW to the form of LDP, and fix a bug of STLXP.

Change-Id: I5d2b51494b92451bf6b072c65cfdd8acf07e9b54
Reviewed-on: https://go-review.googlesource.com/96215
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>