When rewriting loads and stores accessing global variables to use the
GOT we were making use of REGTMP (R10). Unfortunately loads and stores
with large offsets (larger than 20-bits) were also using REGTMP,
causing it to be clobbered and subsequently a segmentation fault.

This can be fixed by using REGTMP2 (R11) for the rewrite. This is fine
because REGTMP2 only has a couple of uses in the assembler (division,
high multiplication and storage-to-storage instructions). We didn't
use REGTMP2 originally because it used to be used more frequently,
in particular for stores of constants to memory. However we have now
eliminated those uses.

This was found while writing a test case for CL 63030. That test case
is included in this CL.

Change-Id: I13956f1f3ca258a7c8a7ff0a7570d2848adf7f68
Reviewed-on: https://go-review.googlesource.com/65011Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>