cmd/link: make symbol data writable before toc fixup

On ppc64le, we need to insert a load to restore the toc
pointer in R2 after calling into plt stubs. Sometimes the
symbol data is loaded into readonly memory. This is the
case when linking with the race detector code.

Likewise, add extra checks to ensure we can, and are
replacing a nop.

Change-Id: Iea9d9ee7a5ba0f4ce285f4d0422823de1c037cb7
Reviewed-on: https://go-review.googlesource.com/c/go/+/304430


Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>