overhaul implementation-internal signal protections

the new approach relies on the fact that the only ways to create
sigset_t objects without invoking UB are to use the sig*set()
functions, or from the masks returned by sigprocmask, sigaction, etc.
or in the ucontext_t argument to a signal handler. thus, as long as
sigfillset and sigaddset avoid adding the "protected" signals, there
is no way the application will ever obtain a sigset_t including these
bits, and thus no need to add the overhead of checking/clearing them
when sigprocmask or sigaction is called.

note that the old code actually *failed* to remove the bits from
sa_mask when sigaction was called.

the new implementations are also significantly smaller, simpler, and
faster due to ignoring the useless "GNU HURD signals" 65-1024, which
are not used and, if there's any sanity in the world, never will be
used.

src/internal/pthread_impl.h

@@ -75,9 +75,12 @@ struct __timer {
 
 #include "pthread_arch.h"
 
-#define SIGCANCEL 32
-#define SIGSYSCALL 33
-#define SIGTIMER 34
+#define SIGTIMER 32
+#define SIGCANCEL 33
+#define SIGSYSCALL 34
+
+#define SIGPT_SET ((sigset_t){{[sizeof(long)==4] = 3<<(32*(sizeof(long)>4))}})
+#define SIGTIMER_SET ((sigset_t){{ 0x80000000 }})
 
 int __set_thread_area(void *);
 int __libc_sigaction(int, const struct sigaction *, struct sigaction *);