It looks like many of these tests are already disabled on emscripten,
which also doesn't seem to support environment variables and subprocess
spawning. Just add a similar tag for CloudABI. While there, sort some of
the lists of operating systems alphabetically.
This commit implements stack probes on x86/x86_64 using the freshly landed
support upstream in LLVM. The purpose of stack probes here are to guarantee a
segfault on stack overflow rather than having a chance of running over the guard
page already present on all threads by accident.
At this time there's no support for any other architecture because LLVM itself
does not have support for other architectures.
