Force MK_CLANG_IS_CC on in XMAKE.

This ensures that a bootstrap clang compiler is always installed as cc
in WORLDTMP. If it is only installed as 'clang' then /usr/bin/cc is
used during the build instead of the bootstrap compiler.

Reviewed by: imp
MFC after: 1 month
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D22332