libc: align wasm32 pthread entry stacks

[brandonpayton]

What this does

#907 already landed PHP support, ABI 39, and the shared process, VFS, host,
package, and test fixes found during that work. This PR does not replay those
changes.

The unique fix here aligns the stack passed to kernel_clone to the 16-byte
boundary expected by wasm32 code generation. Without that alignment, a fresh
pthread can corrupt stack-based variadic arguments. The regression formats the
same %llx%c shape that exposed the problem in SQLite, but tests the libc
contract directly through a new pthread.

What was removed

The old PR mixed this libc fix with SQLite Node/browser test-runner work. That
harness work now belongs to #783 and is deliberately absent here. No mmap,
package, VFS, or unrelated #769 changes remain; the final diff is exactly one
libc source file and one Sortix regression.

ABI and package impact

The kernel_clone arguments and ABI layout are unchanged, so there is no ABI
version bump. Rebuilding musl changes libc/toolchain artifact bytes. No package
revision changes are included.

Validation run

All validation used scripts/dev-shell.sh on exact head
fc25245984f346a5620998f02f93ad9b85dae80b:

• wasm32 musl rebuild passed;
• the focused Sortix pthread regression passed 1/1;
• the ABI snapshot/version check passed;
• the full libc suite passed 303 tests, with 20 expected failures and one known
  flaky test that passed on this run;
• the full POSIX suite passed 174 tests, with 3 expected failures and 2
  supported-boundary skips;
• three initial /bin/sh setup failures passed after the declared dash/rootfs
  dependency was materialized;
• kernel, fork-instrument, program, and host build stages completed.

Validation not run

The full browser and Sortix suites were not run locally. Focused host pthread
Vitest did not start because global setup requires a wasm64 sysroot; wasm64 was
not built because this change is in the wasm32 libc implementation. The full
rootfs source-fallback build was stopped after the relevant artifacts existed.
No performance measurement was made and this PR makes no performance claim.

Extracted from the useful pthread portion of #769 and narrowed from the old
composite #784 diff.

[github-actions]

Phase B-1 matrix build status — pr-784-staging

ABI v39. 71 built, 0 failed, 71 total.

Package	Arch	Status	Sha
icu	wasm32	built	1b708b23
libcurl	wasm32	built	5d2baeb6
libcxx	wasm32	built	10822bd9
libcxx	wasm64	built	25ef3300
libiconv	wasm32	built	c599d681
libpng	wasm32	built	ba7e28f0
libxml2	wasm32	built	d98f801a
libzip	wasm32	built	3dd483d4
openssl	wasm32	built	424389bb
openssl	wasm64	built	912c6480
sqlite	wasm32	built	7a1d359e
sqlite	wasm64	built	d6e76f5f
zlib	wasm32	built	8d1bcd09
zlib	wasm64	built	72d2453b
bc	wasm32	built	b698cee4
bzip2	wasm32	built	cdb19a37
coreutils	wasm32	built	c28d9a8a
curl	wasm32	built	82d34889
dash	wasm32	built	78cb51e3
diffutils	wasm32	built	bb0fe725
dinit	wasm32	built	0dd97701
fbdoom	wasm32	built	197275c8
file	wasm32	built	81d6e9e3
findutils	wasm32	built	2ee580be
gawk	wasm32	built	20093498
git	wasm32	built	d9662860
grep	wasm32	built	54ab22a4
gzip	wasm32	built	98d8b80e
hello	wasm32	built	3894ce0b
kandelo-sdk	wasm32	built	1e680135
kernel	wasm32	built	ddd26fef
less	wasm32	built	82b4d006
lsof	wasm32	built	f615f276
m4	wasm32	built	cf2867c0
make	wasm32	built	c78cf168
mariadb	wasm32	built	d3638d5d
mariadb	wasm64	built	43505255
modeset	wasm32	built	cb65434e
msmtpd	wasm32	built	110af331
nano	wasm32	built	7e849192
ncurses	wasm32	built	5aac1ff2
netcat	wasm32	built	e9929ccd
nginx	wasm32	built	436389b6
php	wasm32	built	2fc5a872
posix-utils-lite	wasm32	built	cd44f9a1
ruby	wasm32	built	fafb2e6d
sed	wasm32	built	d30d7968
spidermonkey	wasm32	built	70375ff9
tar	wasm32	built	150eb5ed
tcl	wasm32	built	214ce39f
unzip	wasm32	built	519079b3
userspace	wasm32	built	29ed6757
vim	wasm32	built	7cab6404
wget	wasm32	built	6a064f94
xz	wasm32	built	7f2d6ad6
zip	wasm32	built	44cbe377
zstd	wasm32	built	de354193
bash	wasm32	built	1b021b0c
mariadb-test	wasm32	built	ebc24b15
mariadb-vfs	wasm32	built	ada20fed
mariadb-vfs	wasm64	built	dbd74217
nethack	wasm32	built	ffc9e23e
node	wasm32	built	60cf998c
spidermonkey-node	wasm32	built	7802aac3
vim-browser-bundle	wasm32	built	e748a6cf
nethack-browser-bundle	wasm32	built	e6d2b836
rootfs	wasm32	built	18d09b37
shell	wasm32	built	2f6265da
lamp	wasm32	built	221369e3
node-vfs	wasm32	built	045bef36
wordpress	wasm32	built	40f60c19

_{Auto-generated; replaced on each push. Raw data in the publish-status workflow artifact.}

[brandonpayton]

Closing this focused source PR because its wasm32 pthread stack-alignment fix and direct regression are now carried by #934 at exact green head 260d75d5adc31b327f5591229dbfbb287d395fd6. #934 keeps them as a separate purpose commit and passed the full staging, libc, and Sortix gates. This is not a claim that this branch was merged verbatim.