srandomdev() seeds the random number generator from /dev/urandom.
The previous stub panicked with todo(""), breaking any program that
calls it on macOS (e.g. SQLite AFP locking path on network volumes).

Seed the shared randomGen from crypto/rand, matching the existing
pattern used by Xarc4random_buf.

Implement Xsrandomdev for darwin

See merge request !31

Xinitstate built a math/rand.Rand and immediately discarded it, so
initstate(seed, ...) was a silent no-op on the non-musl path
(darwin/*bsd/illumos) while the musl/linux build reseeds via srandom.
Seed the shared randomGen under randomMu instead, matching the
just-added Xsrandomdev and bringing the non-musl path in line with musl.

No API change: signature unchanged, still returns NULL, already in the
capi manifests. setstate stays unsupported because random() is modeled
by a single global generator with no per-call state buffer to switch to.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

setstate() returned EINVAL, injecting a spurious error into callers
doing the standard initstate/setstate save/restore idiom. random(3) is
modeled by a single global generator with no independent saved stream
to switch to, so treat setstate as a no-op and return the passed
(non-NULL) pointer to signal success instead of failing.

Also remove the orphaned staticRandomData comment whose only user was
the discarded rand.New call removed in the previous commit.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

wasm2c's --enable-threads output routes every shared-memory load/store
through C11 atomics (the shared memory's data pointer is _Atomic
volatile), including plain f32/f64 access. libc provided the Int/Uint
widths but no float variants, so such a module failed to link with
undefined __atomic_loadFloat32 / __atomic_loadFloat64 /
__atomic_storeFloat32 / __atomic_storeFloat64.

Add the four out-parameter helpers, each delegating to its same-width
integer sibling (an atomic op on the value's bit pattern). wasm has no
atomic-float read-modify-write, so load and store are the complete set.

Verified end-to-end: a shared-memory f32/f64 module translated by
wa2c/wasm2c (--enable-threads) -> ccgo -> Go builds and runs correctly
against this libc.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>