s2: Add arm64 decompression (#324)

Adapted from Snappy equiv. Minor optimizations along the way.
```
enwik9 (stream):
Decompressing. 426854614 -> 1000000000 [234.27%]; 7.618s, 125.2MB/s
Decompressing. 426854614 -> 1000000000 [234.27%]; 4.258s, 224.0MB/s

Blocks:
benchmark                     old ns/op     new ns/op     delta
BenchmarkTwainDecode1e1-4     69.5          63.0          -9.27%
BenchmarkTwainDecode1e2-4     184           153           -16.61%
BenchmarkTwainDecode1e3-4     852           522           -38.74%
BenchmarkTwainDecode1e4-4     30220         14678         -51.43%
BenchmarkTwainDecode1e5-4     570195        231880        -59.33%
BenchmarkTwainDecode1e6-4     3349753       1920724       -42.66%

benchmark                     old MB/s     new MB/s     speedup
BenchmarkTwainDecode1e1-4     143.97       158.69       1.10x
BenchmarkTwainDecode1e2-4     544.75       653.07       1.20x
BenchmarkTwainDecode1e3-4     1174.00      1916.39      1.63x
BenchmarkTwainDecode1e4-4     330.90       681.27       2.06x
BenchmarkTwainDecode1e5-4     175.38       431.26       2.46x
BenchmarkTwainDecode1e6-4     298.53       520.64       1.74x
```

.travis.yml

@@ -4,8 +4,10 @@ os:
   - linux
   - osx
 
+arch:
+  - amd64
+
 go:
-  - 1.13.x
   - 1.14.x
   - 1.15.x
   - 1.16.x
@@ -34,6 +36,14 @@ jobs:
       go: 1.16.x
       script:
         - GOOS=linux GOARCH=386 go test -short ./...
+    - stage: arm64 tests
+      arch: arm64
+      go:
+        - 1.16.x
+      script:
+        - go test -cpu=2 ./s2
+        - go test -cpu=2 -tags=noasm ./s2
+        - go build github.com/klauspost/compress/s2/cmd/s2c && go build github.com/klauspost/compress/s2/cmd/s2d && s2c s2c && s2d s2c.s2 && rm s2c && rm s2d && rm s2c.s2
 
 deploy:
 - provider: script

s2/decode_amd64.s

@@ -34,7 +34,6 @@
 //	- R_TMP0	scratch
 //	- R_TMP1	scratch
 //	- R_LEN	    length or x (shared)
-//	- R_X	    length or x (shared)
 //	- R_OFF	    offset
 //	- R_SRC	    &src[s]
 //	- R_DST	    &dst[d]
@@ -172,13 +171,15 @@ callMemmove:
 	MOVQ R_DST, 24(SP)
 	MOVQ R_SRC, 32(SP)
 	MOVQ R_LEN, 40(SP)
+	MOVQ R_OFF, 48(SP)
 	CALL runtime·memmove(SB)
 
 	// Restore local variables: unspill registers from the stack and
 	// re-calculate R_DBASE-R_SEND.
 	MOVQ 24(SP), R_DST
 	MOVQ 32(SP), R_SRC
 	MOVQ 40(SP), R_LEN
+	MOVQ 48(SP), R_OFF
 	MOVQ dst_base+0(FP), R_DBASE
 	MOVQ dst_len+8(FP), R_DLEN
 	MOVQ R_DBASE, R_DEND