Fix wrong arm64 scaled register format

Arm64 does not have scaled register format, casue snappy test failed for
current go tip:

	$ go version
	go version devel go1.17-24875e3880 Tue Apr 20 15:14:05 2021 +0000 darwin/arm64
	$ go test
	# github.com/golang/snappy
	./encode_arm64.s:385: arm64 doesn't support scaled register format
	./encode_arm64.s:675: arm64 doesn't support scaled register format
	asm: assembly of ./encode_arm64.s failed
	FAIL	github.com/golang/snappy [build failed]

See https://go-review.googlesource.com/c/go/+/289589