Print stack overflow messages for Linux and OS X

This installs signal handlers to print out stack overflow messages on Linux. It also ensures the main thread has a guard page.

This will catch stack overflows in external code. It's done in preparation of switching to stack probes (#16012).

I've done some simple tests with overflowing the main thread, native threads and green threads (with and without UV) on x86-64.
This might work on ARM, MIPS and x86-32.

I've been unable to run the test suite on this because of #16305.
Have you seen this documented somewhere? Looking around [seems](http://www.win.tue.nl/~aeb/linux/lk/lk-5.html) [to](http://en.wikipedia.org/wiki/C_signal_handling) [indicate](http://www.alexonlinux.com/signal-handling-in-linux#registering_signal_handler) [otherwise](http://www.csl.mtu.edu/cs4411.ck/www/NOTES/signal/install.html).