[exception] remove dynamic loading of libunwind/libcorkscrew on Android

[exception] remove dynamic loading of libunwind/libcorkscrew on Android

we will rely on the output of debuggerd in order to get a native stack
trace.  The arguments to support that:

(a) native stack trace by debuggerd is at least as good as the one we get via
    libunwind (most of the time the trace provided by libunwind is useless?)

(b) we cannot screw up by accident (e.g. we have seen issues with a too small altstack)

(c) we don't need to maintain the libunwind integration into mono (which we
    sort of had to do because of the upcoming dlopen limitation in Android 7.0
    Nougat).

(d) the libmonosgen-2.0.so had about one megabyte more footprint (16mb ->
    17mb). I'm talking about a debug build here though.

See mailing list discussion for more information:
http://lists.dot.net/pipermail/android-devel/2016-December/000026.html