Use toolchain to get binary extension for symlinks

Get the binary extension directly from the toolchain instead of
computing it from the target.

Test: mmma -j external/clang
Change-Id: Ib181bc60d0617ad50529791e7b9fc61268376819
diff --git a/cc/binary.go b/cc/binary.go
index 1634a83..7755b5f 100644
--- a/cc/binary.go
+++ b/cc/binary.go
@@ -301,7 +301,7 @@
 	binary.baseInstaller.install(ctx, file)
 	for _, symlink := range binary.Properties.Symlinks {
 		binary.symlinks = append(binary.symlinks,
-			symlink+binary.Properties.Suffix+binary.baseInstaller.path.Ext())
+			symlink+binary.Properties.Suffix+ctx.toolchain().ExecutableSuffix())
 	}
 
 	if binary.Properties.Symlink_preferred_arch {