[php-src] PR #25362: Build with SSE2 & fpmath=sse on x86

Pull Request: https://github.com/php/php-src/pull/25362 Author: marc-mabe SSE2 is part of the x86-64 baseline, but optional on 32-bit x86, where GCC targets i686 by default. Such builds miss the __SSE2__ code paths and compute doubles on the x87 unit. x87 registers have a 15-bit exponent that zend_init_fpu() cannot clamp, so subnormal results are rounded twice and can be one ULP off: $a = 5.678650331314532e-158; $b = 2.3715405351663569e-153; var_dump($a * $b); // x87: 1.34671494457485E-310 // SSE2 and 64-bit: 1.3467149445748E-310 (correctly rounded) The new PHP_CHECK_SSE2 macro in build/php.m4, called from the platform-specific compile settings in configure.ac, prepends to CFLAGS and CXXFLAGS on x86 targets: - -msse2 if supported and not enabled by default; - -mfpmath=sse if SSE2 is enabled and doubles do not use it by default. Clang does this by itself; GCC needs the flag. Configure warns but builds without SSE2 if the build machine cannot run SSE2 code. When cross compiling, -msse2 is added without that check. The resulting build requires SSE2. Pass CFLAGS=-mno-sse2 to build without SSE2. phpize builds do not call the macro, but an extension can call PHP_CHECK_SSE2 from its config.m4.