Support Mac 10.14 SDK

Now that we don't compile 32-bit binaries, this seems to work, at least
in Xcode 10 beta 6.

Test: m checkbuild host
Change-Id: I61a98a3c31b331d6d01cd633f8d949fad86047cb
diff --git a/cc/config/x86_darwin_host.go b/cc/config/x86_darwin_host.go
index 4ec2b31..c97ef4b 100644
--- a/cc/config/x86_darwin_host.go
+++ b/cc/config/x86_darwin_host.go
@@ -88,6 +88,7 @@
 		"10.11",
 		"10.12",
 		"10.13",
+		"10.14",
 	}
 
 	darwinAvailableLibraries = append(