MFC 354661,354693: Fix WITH_CLANG_BOOTSTRAP without WITH_CLANG_IS_CC.

354661:
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.

354693:
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.