`man 3 signal`'s declaration has a face _only a mother could love_.
sighandler_t and __sighandler_t are not defined in the C standard, or POSIX.
They are helpful typedefs provided by glibc and the Linux kernel UAPI headers
respectively since working with function pointers' syntax can be painful. But
we should not rely on them; in C++ we have `auto*` and `using` statements.
Remove the proxy header, and only include a typedef for sighandler_t when
targeting Linux, for compatibility with glibc.
Fixes: #125598
Summary:
This header is practically useless, but we provide it mostly for the
macros so that applications can compile. I'm only doing this for the
`libc++` unittests that want it, and it is part of the C standard
technically. I just made an RPC call to do `raise`. Anything more isn't
going to work since it'd be way too annoying to make the CPU call into
some signal handler the GPU registered.
