`std.debug.cpu_context.signal_ucontext_t` incorrectly uses libc structures

[mlugg]

This type is supposed to be the ucontext_t type which is passed to POSIX signal handlers. However, the parts of the standard library it was taken from were in places mirroring the ucontext_t types in libc, which are often extended versions of the data structure given to signal handlers. The definitions of signal_ucontext_t should be audited and corrected to match the structure populated by the kernel before invoking a signal handler.

[alexrp]

#25516