runtime: use vDSO clock_gettime for time.now & runtime.nanotime on Linux/amd64

Performance improvement aside, time.Now() now gets real nanosecond resolution
on supported systems.

Benchmark done on Core i7-2600 @ 3.40GHz with kernel 3.5.2-gentoo.
original vDSO gettimeofday:
BenchmarkNow    100000000               27.4 ns/op
new vDSO gettimeofday fallback:
BenchmarkNow    100000000               27.6 ns/op
new vDSO clock_gettime:
BenchmarkNow    100000000               24.4 ns/op

R=golang-dev, bradfitz, iant, iant
CC=golang-dev
https://golang.org/cl/6814103

src/pkg/runtime/sys_linux_amd64.s

@@ -102,31 +102,37 @@ TEXT runtime·mincore(SB),7,$0-24
 
 // func now() (sec int64, nsec int32)
 TEXT time·now(SB), 7, $32
+	MOVQ	runtime·__vdso_clock_gettime_sym(SB), AX
+	CMPQ	AX, $0
+	JEQ	fallback_gtod
+	MOVL	$0, DI // CLOCK_REALTIME
+	LEAQ	8(SP), SI
+	CALL	AX
+	MOVQ	8(SP), AX	// sec
+	MOVQ	16(SP), DX	// nsec
+	MOVQ	AX, sec+0(FP)
+	MOVL	DX, nsec+8(FP)
+	RET
+fallback_gtod:
 	LEAQ	8(SP), DI
 	MOVQ	$0, SI
 	MOVQ	runtime·__vdso_gettimeofday_sym(SB), AX
 	CALL	AX
 	MOVQ	8(SP), AX	// sec
 	MOVL	16(SP), DX	// usec
-
-	// sec is in AX, usec in DX
-	MOVQ	AX, sec+0(FP)
 	IMULQ	$1000, DX
+	MOVQ	AX, sec+0(FP)
 	MOVL	DX, nsec+8(FP)
 	RET
 
 TEXT runtime·nanotime(SB), 7, $32
-	LEAQ	8(SP), DI
-	MOVQ	$0, SI
-	MOVQ	$0xffffffffff600000, AX
-	CALL	AX
-	MOVQ	8(SP), AX	// sec
-	MOVL	16(SP), DX	// usec
+	CALL	time·now(SB)
+	MOVQ	0(SP), AX	// sec
+	MOVL	8(SP), DX	// nsec
 
 	// sec is in AX, usec in DX
 	// return nsec in AX
 	IMULQ	$1000000000, AX
-	IMULQ	$1000, DX
 	ADDQ	DX, AX
 	RET
 

src/pkg/runtime/vdso_linux_amd64.c

@@ -161,11 +161,13 @@ static version_key linux26 = { (byte*)"LINUX_2.6", 0x3ae75f6 };
 // initialize with vsyscall fallbacks
 void* runtime·__vdso_time_sym = (void*)0xffffffffff600400ULL;
 void* runtime·__vdso_gettimeofday_sym = (void*)0xffffffffff600000ULL;
+void* runtime·__vdso_clock_gettime_sym = (void*)0;
 
-#define SYM_KEYS_COUNT 2
+#define SYM_KEYS_COUNT 3
 static symbol_key sym_keys[] = {
 	{ (byte*)"__vdso_time", &runtime·__vdso_time_sym },
 	{ (byte*)"__vdso_gettimeofday", &runtime·__vdso_gettimeofday_sym },
+	{ (byte*)"__vdso_clock_gettime", &runtime·__vdso_clock_gettime_sym },
 };
 
 static void vdso_init_from_sysinfo_ehdr(struct vdso_info *vdso_info, Elf64_Ehdr* hdr) {