Expose __builtin_mul_overflow via port_def.inc/port_undef.inc

Follow up to commit b9e056f779f1c6cbf07a7a11be6f5ee6a4d433b8

Not all supported compilers support __has_builtin, so define a protobuf flag
and check that it is defined before trying to use it.

Found on: https://github.com/apple/swift-protobuf/actions/runs/4733817439/jobs/8401873649

PiperOrigin-RevId: 525179304

Thomas Van Lenten committed 3y ago

80f0f7463d77186885762080c93d6cc99062e045

Parent: c32f8e3

Committed by Copybara-Service <copybara-worker@google.com> on 4/18/2023, 5:09:42 PM