Posix RTS test fixes and Android compatibility

Posix RTS test fixes and Android compatibility

Android's libc (bionic) defines the _GNU_SOURCE macro but fails to adhere
to the GNU sematics in (at least) one case - the strerror_r function.
The function follows XSI semantics instead of the GNU one which caused our
code to segfault on Android. This patch special-cases Android to use the XSI
semantics for strerror_r. This un-breaks some POSX real-time singals tests.

It also makes provisions for detecting whether the real-time signals are safe
to use on the current platform. They are safe for all platforms desktop Mono
runs on, but they're not safe on most Android platforms.