cmd/compile: use STP/LDP around morestack on arm64
The spill/restore code around morestack is almost never exectued, so
we should make it as small as possible. Using 2-register loads/stores
makes sense here. Also, the offsets from SP are pretty small so the
offset almost always fits in the (smaller than a normal load/store)
offset field of the instruction.
Makes cmd/go 0.6% smaller.
Change-Id: I8845283c1b269a259498153924428f6173bda293
Reviewed-on: https://go-review.googlesource.com/c/go/+/621556
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>