runtime: don't use R11 in nanotime1/walltime1 on ARM

R11 a.k.a. REGTMP is the temp register used by the assembler. It
may be clobbered if the assembler needs to synthesize
instructions. In particular, in nanotime1/walltime1, the load of
global variable runtime.iscgo clobbers it. So, avoid using R11
to hold a long-lived value.

Fixes #36309.

Change-Id: Iec2ab9d664532cad8fbf58da17f580e64a744f62
Reviewed-on: https://go-review.googlesource.com/c/go/+/212641


Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Andrew G. Morgan <agm@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>