Refine the Android-specific GCC compatibility flags.

To pass all existing clang test cases, we should not
change the default of -fno-cxx-missing-return-semantics
and -fno-global-ctor-const-promotion in non-Android
environment.

In this CL, these compatibility flags will only be
enabled by default for Android.  Besides, several test
cases are added.

Change-Id: I8e9a7f453ccc0f2cae21d24aba598844b2b2767f