Use __builtin_unreachable() for ZEND_UNREACHABLE

[iluuu1994]

__builtin_assume() is only supported on Clang. __builtin_unreachable() may provide additional hints to GCC.

More accurately, on GCC the code if (__builtin_expect(!(0), 0)) __builtin_unreachable(); is generated for unreachable. However, GCC does not seem to accurately mark the path as unreachable due to the if statement. Removing it fixes the warning.

Motivated by the compile error in https://github.com/php/php-src/actions/runs/6242795686/job/16947298463.

/cc @TimWolla

[TimWolla]

Apparently ZEND_ASSUME() already has a variant that is based on __builtin_unreachable, but the updated semantics clearly make sense.

With regard to the updated implementation of php_round_helper, it will need an update anyway, because nothing currently validates the rounding mode, thus round(0, mode: 9) crashes.

[iluuu1994]

Right, I didn't look closely enough. For some reason, if (__builtin_expect(!(0), 0)) __builtin_unreachable(); doesn't make GCC happy, but __builtin_unreachable() alone does. assert(0); doesn't work either. I'm not sure if it makes a difference in the actual code generation.