tools/build/make.py: fix cross build on Fedora Linux

Fedora defines shell functions for some commands used by FreeBSD build
scripts. Unortunatelly it makes them behave incorrectly for our purposes.

For instance 'which which' returns something like:

which ()
{

( alias;
eval ${which_declare} ) | /usr/bin/which --tty-only --read-alias ...

}

instead of

/usr/bin/which

This patch unsets those functions to restore original/expected behavior

Reviewed by: emaste, imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36900