Refine r354661 to unbreak the GCC_BOOTSTRAP case.

MK_CLANG_IS_CC controls installing links for GCC, not just clang. Set
MK_CLANG_IS_CC to the value of MK_CLANG_BOOTSTRAP. This will leave it
as "no" if no bootstrap compiler is being built or GCC 4.2.1 is being
used as the bootstrap compiler, and "yes" if clang is being used as
the bootstrap compiler.

Submitted by: bdrewery (kind of, he suggested this on IRC while I was

	  		  testing the original patch)

Reviewed by: kevans, imp
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D22350