Force MK_CLANG_IS_CC on in XMAKE.

[jhb]

Summary

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.

Test Plan

• tried to build riscv64sf with WITH_CLANG_BOOTSTRAP=yes but without WITH_CLANG_IS_CC=yes

[imp]

Hmmm, does this break native builds for gcc?

[jhb]

It shouldn't. The only thing that honors MK_CLANG_IS_CC is the clang Makefile, and during XMAKE we only enter that subdirectory if MK_CLANG_BOOSTRAP is yes, and we shouldn't have both that and MK_GCC_BOOTSTRAP as yes at the same time.