cmd/compile, runtime, etc: get rid of constant FP registers

cmd/compile, runtime, etc: get rid of constant FP registers

On ARM64, MIPS64, and PPC64, some floating point registers were
reserved for constants 0, 1, 2, 0.5, etc. This CL removes them.

On ARM64, they are never used. On MIPS64 and PPC64, the only use
case is a multiplication-by-2 in the old backend of the compiler,
which is replaced with an addition.

Change-Id: I737cbf43283756e3408964fc88c567a938c57036
Reviewed-on: https://go-review.googlesource.com/28095
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>