Use a non RT signal on Android since its userland is not very keep to it.

	* threads.c: On Android we now use SIGUNUSED instead of RT2 since bionic
	exposes a 32bits sigset_t which cannot accommodate signals in the RT range.

	The alternative would be to directly syscall the kernel and use the RT range,
	but that would mean patch all functions that deal with sigset_t to use a wrapper.
	So, given the effort/result of it on this case. It makes more sense to just
	use an unused signal for that.

	The linux kernel has 2 syscall APIs for signals, the regular one and the rt
	variant. EG, we have sys_sigprocmask and sys_rt_sigprocmask.

	The regular one works with 32bits sigset_t while the rt_ variant deals with
	64bits (128bits on mips) sigset_t. This is, I believe, largely due to legacy
	support for old glibc when they added support for RT signals in a very very
	distant past - before the kernel git epoch.

	Said that, bionic, for some reason, decided to go with the old one. Android's
	kernel has full support for RT signals, which makes the whole thing even more
	interesting. I could not figure out why it ended up like that, probably some
	historical accident that was never noticed.

	Current bionic (Jelly Bean 4.2.1) function that operate on sigset_t don't
	perform any bounds check which, when using signal 33, leads to memory corruption.
	On our case it leads to stack corruption and random crashes.
Make mono_print_unhandled_exception resilient to nested exceptions.

	* object.c (mono_print_unhandled_exception): Ensure that nested exceptions are
	properly catch not leading to a stack overflow of recursive fail.
Move mono_runtime_printf_err to use critical level as error results in an abort.
Improve our native crash experience.

	* mini-exceptions.c (mono_invoke_unhandled_exception_hook): Print something
	even if we fail to produce a backtrace.

	* mini-exceptions.c (mono_handle_native_sigsegv): Use exit on android
	since abort triggers another segv. Always Print the fatal native error
	even on "special" targets that have no backtrace symbols support.