[PHP-wasm] Add support for concurrent popen calls

[bgrgicak]

Motivation for the change, related issues

The previous popen/pclose implementation used single global variables (wasm_popen_last_pid and wasm_pclose_ret) to track the spawned process PID and exit code. This meant concurrent writable popen() calls would clobber each other's state, producing wrong exit codes or closing the wrong process.

Additionally, popen/pclose interception relied on a VCWD_POPEN macro patch and explicit sed replacements in mail.c, which was fragile and didn't cover all call sites.

Implementation details

Linker-level wrapping replaces macro patching

Instead of patching VCWD_POPEN in zend_virtual_cwd.h and sed-replacing popen/pclose in mail.c, this PR uses --wrap=popen and --wrap=pclose linker flags. This transparently redirects all popen()/pclose() calls in the compiled C code to __wrap_popen/__wrap_pclose, which delegate to wasm_popen/wasm_pclose. For pclose, handles not created by wasm_popen fall through to the real libc pclose.

Per-fd PID tracking instead of globals

Replaces wasm_popen_last_pid and wasm_pclose_ret with an fd-to-pid mapping stored in the existing JS PHPWASM.processTable. Three new Emscripten library functions manage the mapping:

• js_popen_set_pid_for_fd(fd, pid) — called by wasm_popen after spawning a process
• js_popen_get_pid_for_fd(fd) — called by wasm_pclose to find which process to wait on
• js_popen_clear_pid_for_fd(fd) — called by wasm_pclose before closing the file

This allows multiple popen handles to be open simultaneously without state corruption.

Emscripten library updates

Adds __wrap_popen, __wrap_pclose, and zif_pclose to the Asyncify import list and JSPI export list so the new wrapper functions can suspend correctly.

TODO

• Recompile all PHP versions after PR review

Testing Instructions

• CI