configure: disable Apple clang size optimizations that hurt runtime speed

[staabm]

disclaimer: this change was generated by claude opus. I have little experience with php-src development

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Apple clang enables the AArch64 machine outliner and hot/cold code splitting by default at -O2. Both trade speed for size: the outliner inserts extra bl/ret pairs into hot paths (measured inside the inlined zval destructor loops of zend_array_destroy, among others), and cold-split fragments (4700+ in a default cli build) are compiled for size, which mispredicted branch hints turn into slow hot code.

Disabling both speeds up a CPU-bound static analysis workload (PHPStan analysing its own codebase, no opcache) by ~4% wall time on an Apple M-series machine. The flags are added only when the compiler accepts them (-Werror guards against clang's unknown -m flag warnings), so non-Apple toolchains are unaffected.

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benchmark (3 runs after 2 warmup runs):

run /Users/staabm/workspace/php-src/sapi/cli/php bin/phpstan clear-result-cache -q && /Users/staabm/workspace/php-src/sapi/cli/php -d memory_limit=450M bin/phpstan -v after checking out phpstan/phpstan-src#5942 from the git root folder.
(on first time phpstan-src checkout you need composer install and make to prepare the codebase)

before this PR:
14,47s
14,87s
14,60s

after this PR:
13,91s
13,68s
13,86s

on M4-Pro with macOS 26.6 (25G72)

[realFlowControl]

I played with this PR locally on my M4 Max and can confirm the speedup:

Metric	Baseline (master)	PR (-mno-outline -Xclang -fno-split-cold-code)
Mean	8.485s	8.194s
Median	8.446s	8.213s
Stdev	0.059s	0.058s
Speedup	—	3.4%

With this setup:

• Compiler: Apple clang 21.0.0 (clang-2100.1.1.101)
• PHP: 8.6.0-dev, built from same commit (e9deb0a), default ./configure
• PHPStan 2.2.5, level 6, analysing Sylius src/ (v2.2.7-dev)
• 2 warmup runs + 5 alternating timed runs, result cache cleared before each run

One interesting caveat: When compiling PHP with clang from Homebrew I would not see any difference because:

• -mno-outline is accepted but is a no-op: upstream LLVM (Homebrew) does not enable the AArch64 machine outliner by default at -O2, unlike Apple clang.
• -Xclang -fno-split-cold-code is specific to Apple clang and the Homebrew version rejects those flags.