The assembler back end uses F15 as a temporary register in these
instructions.

Checked the assembler back end and made sure that this is the
only case clobbering F15.

Fixes #19403.

Change-Id: I02b9e00fdd9229db899f501c8e9b306e02912d83
Reviewed-on: https://go-review.googlesource.com/37792
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>