runtime/cgo: fix crosscall2 on ppc64x

Some uses of crosscall2 did not work on ppc64le and probably
aix-ppc64. In particular, if there was a main program compiled
with -buildmode=pie and used a plugin which invoked crosscall2,
then failures could occur due to R2 getting set incorrectly along the
way. The problem was due to R2 being saved on the caller's
stack; it is now saved on the crosscall2 stack. More details can be
found in the issue.

This adds a testcase where the main program is built with pie
and the plugin invokes crosscall2.

This also changes the save of the CR bits from MOVD to MOVW as
it should be.

Fixes #43228

Change-Id: Ib5673e25a2ec5ee46bf9a1ffb0cb1f3ef5449086
Reviewed-on: https://go-review.googlesource.com/c/go/+/319489


Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Trust: Heschi Kreinick <heschi@google.com>