All the existing code that manipulates `ucontext_t` expects there to be a
glibc-compatible sigmask (1024-bit).  The `ucontext_t` struct need to be
cleaned up so the glibc-dependent format is only used when linking
glibc/musl library, but that is a more involved change.

In practice, no Zig code looks at the sigset field contents, so it just
needs to be the right size.