cross-build: Workaround system-provided strchrnul on macOS 15.4+

macOS added a native strchrnul in 15.4. There is not an easy way to
detect it at compile time, so use a macro to rename our local inline
version to avoid conflicts while also forcing its use during
bootstrap. The local version can be removed once macOS versions older
than 15.4 are no longer supported as build hosts.

Co-authored by: jrtc27
Reported by: kib
Reviewed by: jrtc27
Differential Revision: https://reviews.freebsd.org/D49893