cross-build: Define DEFFILEMODE and ALLPERMS if not defined

GLIBC defines these, but MUSL does not. FreeBSD's bootstrap code uses
these defines, so define them if they aren't yet defined.

Co-authored-by: Val Packett <val@packett.cool>
Sponsored by: Netflix
Pull Request: https://github.com/freebsd/freebsd-src/pull/1066
Differential Revision: https://reviews.freebsd.org/D45375