emscripten: fix pthread type sizes for wasm64 (MEMORY64)

[gergelyvagujhelyi]

On wasm64-unknown-emscripten (MEMORY64) several pthread types are larger than
the hardcoded wasm32 sizes used here, so pthread_attr_init overruns the
pthread_attr_t that std stack-allocates in Thread::new, corrupting the
result and making std::thread::spawn fail with a spurious error. This makes
the affected sizes pointer-width-aware.

The numbers are sizeof() from emscripten's own headers, compiled both ways:

#include <pthread.h>
#include <stdio.h>
int main(void) {
    printf("attr=%zu mutex=%zu rwlock=%zu cond=%zu\n",
           sizeof(pthread_attr_t), sizeof(pthread_mutex_t),
           sizeof(pthread_rwlock_t), sizeof(pthread_cond_t));
}

emcc              -> attr=44 mutex=24 rwlock=32 cond=48
emcc -sMEMORY64=1 -> attr=88 mutex=40 rwlock=56 cond=48

• pthread_attr_t is exactly 11 pointer-width words (44 = 11·4, 88 = 11·8),
  so [usize; 11] matches both with no cfg — and is identical to the previous
  [u32; 11] on wasm32.
• pthread_mutex_t / pthread_rwlock_t aren't a fixed multiple of the pointer
  width (musl sizes them as int __i[N] with a different N per width), so they
  need explicit per-width values.
• pthread_cond_t (48) and pthread_t = c_ulong (4/8 bytes) are already correct.

Found while bringing up wasm64-unknown-emscripten downstream (custom target
spec + -Zbuild-std): with the wasm32 sizes, pthread_create reports success at
the emscripten layer but std reads a corrupted result and panics
failed to spawn thread, so threads can't be spawned at all.

There's no wasm64-unknown-emscripten job in CI (it isn't a built-in target), so
this isn't exercised by libc-test; the sizes above are reproducible with the
snippet.