runtime: check pointer equality in arm64 cmpbody

Updates #11336

Follow the lead of amd64 by doing a pointer equality check
before comparing string/byte contents on arm64.

BenchmarkCompareBytesEqual-8               25.8           26.3           +1.94%
BenchmarkCompareBytesToNil-8               9.59           9.59           +0.00%
BenchmarkCompareBytesEmpty-8               9.59           9.17           -4.38%
BenchmarkCompareBytesIdentical-8           26.3           9.17           -65.13%
BenchmarkCompareBytesSameLength-8          16.3           16.3           +0.00%
BenchmarkCompareBytesDifferentLength-8     16.3           16.3           +0.00%
BenchmarkCompareBytesBigUnaligned-8        1132038        1131409        -0.06%
BenchmarkCompareBytesBig-8                 1126758        1128470        +0.15%
BenchmarkCompareBytesBigIdentical-8        1084366        9.17           -100.00%

Change-Id: Id7125c31957eff1ddb78897d4511bd50e79af3f7
Reviewed-on: https://go-review.googlesource.com/13885Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/asm_arm64.s

@@ -870,6 +870,8 @@ TEXT bytes·Compare(SB),NOSPLIT,$-4-56
 // On exit:
 // R4, R5, and R6 are clobbered
 TEXT runtime·cmpbody<>(SB),NOSPLIT,$-4-0
+	CMP	R2, R3
+	BEQ	samebytes // same starting pointers; compare lengths
 	CMP	R0, R1
 	CSEL    LT, R1, R0, R6 // R6 is min(R0, R1)