fix(ci): parallelize the clang-cl Windows lane with /MP

[luthermonson]

Problem

build-windows-clang ("Windows (x86_64, clang-cl TAILCALL, experimental)") times out. A real run attached its runner fine, got through every setup step, and was cancelled at exactly ~2h during Build php8embed (clang-cl toolset) — the PHP compile itself. The MSVC lane (build-windows) fits under 2h and publishes its assets; only the clang-cl lane overruns.

Root cause

PHP's win32 build enables multi-process compilation (/MP) only inside if (VS_TOOLSET) in win32/build/config.w32:

if (VS_TOOLSET) {
    ARG_WITH('mp', ...'auto');
    ...
    ADD_FLAG('CFLAGS', ' /MP ');   // multi-process
}

This lane patches PHP_TOOLSET = "clang", so VS_TOOLSET is false and that entire block is skipped. PHP's batched inference rule (confutils.js, the non-PHP_MP_DISABLED branch) still hands many .c files to a single clang-cl invocation, but with no /MP that invocation compiles them single-process. clang-cl is slower per TU than MSVC, so the serial batch overruns the 2h ephemerd VM job_timeout. The MSVC lane fits precisely because it gets PHP's default /MP auto for free.

Fix

Extend the existing "Clone and patch php-src" step to add /MP to CFLAGS for the non-VS toolset, right after toolset_setup_common_cflags() (where CFLAGS is defined and VS_TOOLSET is already set):

if (!VS_TOOLSET && PHP_DEBUG != "yes") {
    ADD_FLAG("CFLAGS", " /MP ");
    STDOUT.WriteLine("Enabling multi process build (clang-cl, php-sdk lane)");
}

• clang-cl honors /MP (this lane pins LLVM 22.x; /MP support landed long before).
• Mirrors the VS lane's /MP auto (all cores) and keeps PHP's own debug-build carve-out.
• Hard-fails if the toolset_setup_common_cflags(); anchor drifts, matching the other two patches in that step.
• /MP reaches the compile because spc's --no-strip embed path reads CFLAGS= from the generated Makefile and re-passes it to nmake; PHP's configure writes the ADD_FLAG result into that line.

Only comments and the patch step change. No behavior change to the MSVC lane.

Validation

Dispatched a test build from this branch with release_tag_suffix=-clangpar so it uploads to v8.5.7-clangpar and cannot touch production v8.5.7 assets. Not merging until that run confirms the compile fits under 119 min.

[ephpm]

ePHPm Preview — deployed (health check pending)

URL	https://ephpm-php-sdk-pr-67.preview.ephpm.dev
Framework	PHP
PHP	8.5
Deployed in	64.4s

Preview updates automatically on each push to this PR.