Makefile.inc1: Enable requesting the universe toolchain.

make universe builds a cross toolchain under HOST_OBJTMP/tmp via the
universe-toolchain target. However, doing a plain 'make buildworld'
after a universe/tinderbox run (e.g. to reproduce a failure and test
the fix for it), will try to build a new cross toolchain under
OBJTMP/tmp which can be tedious. This commit adds a make variable
(UNIVERSE_TOOLCHAIN) which can be used similar to CROSS_TOOLCHAIN to
request an external toolchain. If this variable is set (value doesn't
matter), the the universe toolchain is used as an external toolchain.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D40777