runtime: fix build for race detector

This CL only fixes the build, there are two failing tests:
RaceMapBigValAccess1 and RaceMapBigValAccess2
in runtime/race tests. I haven't investigated why yet.

Updates #9516.

Change-Id: If5bd2f0bee1ee45b1977990ab71e2917aada505f
Reviewed-on: https://go-review.googlesource.com/2401Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/race_amd64.s

@@ -202,6 +202,9 @@ TEXT	sync∕atomic·LoadUint64(SB), NOSPLIT, $0-0
 TEXT	sync∕atomic·LoadUintptr(SB), NOSPLIT, $0-0
 	JMP	sync∕atomic·LoadInt64(SB)
 
+TEXT	sync∕atomic·LoadPointer(SB), NOSPLIT, $0-0
+	JMP	sync∕atomic·LoadInt64(SB)
+
 // Store
 TEXT	sync∕atomic·StoreInt32(SB), NOSPLIT, $0-0
 	MOVQ	$__tsan_go_atomic32_store(SB), AX