remove explicit locking to prevent __synccall setuid during posix_spawn

for the duration of the vm-sharing clone used by posix_spawn, all
signals are blocked in the parent process, including
implementation-internal signals. since __synccall cannot do anything
until successfully signaling all threads, the fact that signals are
blocked automatically yields the necessary safety.

aside from debloating and general simplification, part of the
motivation for removing the explicit lock is to simplify the
synchronization logic of __synccall in hopes that it can be made
async-signal-safe, which is needed to make setuid and setgid, which
depend on __synccall, conform to the standard. whether this will be
possible remains to be seen.
always block signals in pthread_exit before decrementing thread count

the thread count (1+libc.threads_minus_1) must always be greater than
or equal to the number of threads which could have application code
running, even in an async-signal-safe sense. there is at least one
dangerous race condition if this invariant fails to hold: dlopen could
allocate too little TLS for existing threads, and a signal handler
running in the exiting thread could claim the allocated TLS for itself
(via __tls_get_addr), leaving too little for the other threads it was
allocated for and thereby causing out-of-bounds access.

there may be other situations where it's dangerous for the thread
count to be too low, particularly in the case where only one thread
should be left, in which case locking may be omitted. however, all
such code paths seem to arise from undefined behavior, since
async-signal-unsafe functions are not permitted to be called from a
signal handler that interrupts pthread_exit (which is itself
async-signal-unsafe).

this change may also simplify logic in __synccall and improve the
chances of making __synccall async-signal-safe.
add comments on some of the pthread_exit logic
use atomic decrement rather than cas in pthread_exit thread count

now that blocking signals prevents any application code from running
while the last thread is exiting, the cas logic is no longer needed to
prevent decrementing below zero.
make last thread's pthread_exit give exit(0) a consistent state

the previous few commits ended up leaving the thread count and signal
mask wrong for atexit handlers and stdio cleanup.
fix clobbering of signal mask when creating thread with sched attributes

this was simply a case of saving the state in the wrong place.
synccall signal handler need not handle dead threads anymore

they have already blocked signals before decrementing the thread
count, so the code being removed is unreachable in the case where the
thread is no longer counted.
prevent code from running under a thread id which already gave ESRCH
optimize/debloat raise

use __syscall rather than syscall when failure is not possible or not
to be considered.
transition to using functions for internal signal blocking/restoring

there are several reasons for this change. one is getting rid of the
repetition of the syscall signature all over the place. another is
sharing the constant masks without costly GOT accesses in PIC.

the main motivation, however, is accurately representing whether we
want to block signals that might be handled by the application, or all
signals.