[dev.ssa] cmd/compile: implement Zero, Move, Copy for SSA on ARM

[dev.ssa] cmd/compile: implement Zero, Move, Copy for SSA on ARM

Generate load/stores for small zeroing/move, DUFFZERO/DUFFCOPY for
medium zeroing/move, and loops for large zeroing/move.

cmd/compile/internal/gc/testdata/{copy_ssa.go,zero_ssa.go} tests
passed.

Progress on SSA backend for ARM. Still not complete. A few packages
in the standard library compile and tests passed, including
container/list, hash/crc32, unicode/utf8, etc.

Updates #15365.

Change-Id: Ieb4b68b44ee7de66bf7b68f5f33a605349fcc6fa
Reviewed-on: https://go-review.googlesource.com/23097
Reviewed-by: Keith Randall <khr@golang.org>