misc/cgo/test: skip issue3261 test on darwin/arm64

Because there is no libgcc.

Change-Id: I3b3f80791a1db4c2b7318f81a115972cd2237f07
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/8786Reviewed-by: David Crawshaw <crawshaw@golang.org>

misc/cgo/test/issue3261.go

@@ -13,6 +13,12 @@ int vabs(int x) {
 	puts("testLibgcc is disabled on ARM because 5l cannot handle thumb library.");
 	return (x < 0) ? -x : x;
 }
+#elif defined(__arm64__) && defined(__clang__)
+#include <stdio.h>
+int vabs(int x) {
+	puts("testLibgcc is disabled on ARM64 with clang due to lack of libgcc.");
+	return (x < 0) ? -x : x;
+}
 #else
 int __absvsi2(int); // dummy prototype for libgcc function
 // we shouldn't name the function abs, as gcc might use