MinGW (Windows) builds do not support Clang

Use the default for host builds: linux and darwin use clang, windows
will use gcc.

This is the same behavior as before I rewrote the windows support, where
we were explicitly disabling clang in the build system for windows
builds. I'm making the build error out when setting LOCAL_CLANG := true
on windows modules instead of overriding the selection in the build
system so that it will be easier to gradually test clang for windows in
the future.

Change-Id: I7b293923cf5fc8f066c848179e6f025136d105a3
diff --git a/Android.mk b/Android.mk
index 2130df2..a9f8760 100644
--- a/Android.mk
+++ b/Android.mk
@@ -357,7 +357,6 @@
 
 LOCAL_MODULE := libcompiler_rt-extras
 LOCAL_CFLAGS := $(libcompiler_rt_common_CFLAGS)
-LOCAL_CLANG := true
 LOCAL_SRC_FILES := $(libcompiler_rt_extras_SRC_FILES)
 LOCAL_SANITIZE := never
 LOCAL_MULTILIB := both