sigaltstack and cygwin

[Stephen Weeks]

Cygwin has no sigaltstack.  So, it seems to me that I need to change
the x86-codegen so that %esp is reserved to hold the C stack pointer
when targeting cygwin.  Does that seem right?  Easy?

On a related note, the above restriction, and the similar one that we
use for linux (%esp holds either the C stack pointer or frontier)
should only be imposed on programs that install signal handlers.  I
think that is an important optimization that we should put in.