cmd/compile: make LR allocatable in non-leaf functions on ARM

cmd/compile: make LR allocatable in non-leaf functions on ARM

The mechanism is initially introduced (and reviewed) in CL 30597
on S390X.

Reduce number of "spilled value remains" by 0.4% in cmd/go.

Disabled on ARMv5 because LR is clobbered almost everywhere with
inserted softfloat calls.

Change-Id: I2934737ce2455909647ed2118fe2bd6f0aa5ac52
Reviewed-on: https://go-review.googlesource.com/32178
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>