build: Various remaining cleanups to makefiles.

-fexceptions
    Moved to LOCAL_CPP_FEATURES := exceptions
    From android-ndk-r8e/docs/ANDROID-MK.html:
    "It is recommended to use this variable instead of enabling -frtti
    and -fexceptions directly in your LOCAL_CPPFLAGS definition."

-lz
    Moved from LOCAL_LDLIBS (gles2rice) to LOCAL_EXPORT_LDLIBS (png)
    Again, from android-ndk-r8e/docs/ANDROID-MK.html, the recommended
    way for a static lib (i.e. png) to let its clients know what system
    libraries (i.e. zlib) they must link with is to use the flag
    LOCAL_EXPORT_LDLIBS.

-shared
    Not needed because the NDK toolchain already does that.

-llog
    Not needed by core.

-Wno-psabi
    Just a warning disabler that I missed in commit
    f0d45b8c00f5d6605301e7feaf72dd34bc3549f3

Only include the relevant source files for SDL rendering (i.e. omit
opengl, directx, etc.).  Just saves a bit of compile time.