Fix mac build

Darwin doesn't support --start-group/--end-group, and groups
libraries anyways.

Test: m -j
Change-Id: I0fdbe24973cad3cd84164cdfeced2507a4c6ee91
diff --git a/cc/builder.go b/cc/builder.go
index d0b21ed..c9a6722 100644
--- a/cc/builder.go
+++ b/cc/builder.go
@@ -466,11 +466,11 @@
 		}
 	}
 
-	if flags.groupStaticLibs && len(staticLibs) > 0 {
+	if flags.groupStaticLibs && !ctx.Darwin() && len(staticLibs) > 0 {
 		libFlagsList = append(libFlagsList, "-Wl,--start-group")
 	}
 	libFlagsList = append(libFlagsList, staticLibs.Strings()...)
-	if flags.groupStaticLibs && len(staticLibs) > 0 {
+	if flags.groupStaticLibs && !ctx.Darwin() && len(staticLibs) > 0 {
 		libFlagsList = append(libFlagsList, "-Wl,--end-group")
 	}