this function is mainly (purely?) for obtaining stack address
information, but we also provide the detach state since it's easy to
do anyway.

the issue at hand is that many syscalls require as an argument the
kernel-ABI size of sigset_t, intended to allow the kernel to switch to
a larger sigset_t in the future. previously, each arch was defining
this size in syscall_arch.h, which was redundant with the definition
of _NSIG in bits/signal.h. as it's used in some not-quite-portable
application code as well, _NSIG is much more likely to be recognized
and understood immediately by someone reading the code, and it's also
shorter and less cluttered.

note that _NSIG is actually 65/129, not 64/128, but the division takes
care of throwing away the off-by-one part.

this bug seems to have been around a long time.

this bug was introduced when support for application-provided stacks
was originally added.

the main goal of these changes is to address the case where an
application provides a stack of size N, but TLS has size M that's a
significant portion of the size N (or even larger than N), thus giving
the application less stack space than it expected or no stack at all!

the new strategy pthread_create now uses is to only put TLS on the
application-provided stack if TLS is smaller than 1/8 of the stack
size or 2k, whichever is smaller. this ensures that the application
always has "close enough" to what it requested, and the threshold is
chosen heuristically to make sure "sane" amounts of TLS still end up
in the application-provided stack.

if TLS does not fit the above criteria, pthread_create uses mmap to
obtain space for TLS, but still uses the application-provided stack
for actual call frame stack. this is to avoid wasting memory, and for
the sake of supporting ugly hacks like garbage collection based on
assumptions that the implementation will use the provided stack range.

in order for the above heuristics to ever succeed, the amount of TLS
space wasted on POSIX TSD (pthread_key_create based) needed to be
reduced. otherwise, these changes would preclude any use of
pthread_create without mmap, which would have serious memory usage and
performance costs for applications trying to create huge numbers of
threads using pre-allocated stack space. the new value of
PTHREAD_KEYS_MAX is the minimum allowed by POSIX, 128. this should
still be plenty more than real-world applications need, especially now
that C11/gcc-style TLS is now supported in musl, and most apps and
libraries choose to use that instead of POSIX TSD when available.

at the same time, PTHREAD_STACK_MIN has been decreased. it was
originally set to PAGE_SIZE back when there was no support for TLS or
application-provided stacks, and requests smaller than a whole page
did not make sense. now, there are two good reasons to support
requests smaller than a page: (1) applications could provide
pre-allocated stacks smaller than a page, and (2) with smaller stack
sizes, stack+TLS+TSD can all fit in one page, making it possible for
applications which need huge numbers of threads with minimal stack
needs to allocate exactly one page per thread. the new value of
PTHREAD_STACK_MIN, 2k, is aligned with the minimum size for
sigaltstack.

this should generate faster and smaller code, especially with inline
syscalls. the conditional with cnt is ugly, but thankfully cnt is
always a constant anyway so it gets evaluated at compile time. it may
be preferable to make separate __wake and __wakeall macros without a
count argument.

priv flag is not used yet; private futex support still needs to be
done at some point in the future.

these should have little/no practical impact but they're needed for
strict conformance.

sigsetjmp: store temporaries in jmp_buf rather than on stack.

it's essential to decrement the stack pointer before writing to new
stack space, rather than afterwards. otherwise there is a race
condition during which asynchronous code (signals) could clobber the
data being stored.

it may be possible to optimize the code further using stwu, but I
wanted to avoid making any changes to the actual stack layout in this
commit. further improvements can be made separately if desired.