[FEATURE REQUEST] Implement gcc_jit_sig_atomic_type.

[Marc Nieper-Wißkirchen]

Each target of gcc defines `SIG_ATOMIC_TYPE', which is then used to
implement the built-in preprocessor macro `__SIG_ATOMIC_TYPE__', which
is then used by <signal.h> to typedef `sig_atomic_t'.

This type is used, for example, when writing POSIX-compliant signal handlers.

It is defined as: "An integer type which can be accessed as an atomic
entity even in the presence of asynchronous interrupts made by
signals."

This is a feature request to expose this type to libgccjit by
providing the following function:

gcc_jit_type *gcc_jit_sig_atomic_type (gcc_jit_context *ctxt)

-- Marc

P.S.: Depending on my spare time, I may be able to write a patch that
would implement this feature request.