Respect configured -march when building libargon2

[henderkes]

Summary

Argon2's Makefile defaults OPTTARGET to native, which can compile host-specific instructions into redistributable binaries. Use the last -march in the library's configured CFLAGS. If none is set, pass OPTTARGET=none to select Argon2's portable implementation. Apply the same target to clean, build, and install.

This addresses the Argon2 SIGILL reported in php/frankenphp#2684. The same fix is already on feat/pgo-v3.

Verification

• php -l src/Package/Library/libargon2.php
• Built the pinned Argon2 source with OPTTARGET=none and ran its test suite successfully on aarch64.
• Built it with -march=armv8-a / OPTTARGET=armv8-a and ran the test suite successfully.
• Checked extraction of x86-64 and Arm -march values, multiple flags, and the unset fallback.

[github-actions]

StaticPHP Test Bot

Detected: Extensions: none | Libraries: libargon2 | Targets: none
Active labels: test/tier2
Available labels: need-test (gate), test/linux test/windows test/macos (platform), test/tier2 (extra arch), test/php-83 test/php-84 (PHP version), sapi/frankenphp (extra FrankenPHP SAPI build)
Config: Linux x86_64 + Windows x86_64 + macOS arm64 + Linux aarch64 (Tier2) + macOS x86_64 (Tier2) | PHP 8.5 NTS