Respect MK_TCSH with build-tools and native-xtools

This helps reduce the WORLDTMP footprint slightly.

Based on a patch I submitted 5 years ago to GNATS.

PR: 174051
MFC after: 2 weeks
Relnotes: yes (anyone who cross-builds with MK_TCSH=yes will run into

		     build failures if the host doesn't have tcsh(1))

Reminded by: Fabian Keil <fk@fabiankeil.de>