[libc][signal] clean up usage of sighandler_t (#125745)
`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
