[runtimes] Shrink the set of runtimes included in the bootstrapping build by default

This patch is in preparation for removing libcxx, libcxxabi and libunwind
from LLVM_ENABLE_PROJECTS. When we make that switch, folks who were
previously using LLVM_ENABLE_PROJECTS=all in order to build those
runtimes will be able to add LLVM_ENABLE_RUNTIMES=all to start building
those projects using the bootstrapping build.

This is technically a breaking change for folks who had been using
LLVM_ENABLE_RUNTIMES=all, however I suspect the set of people currently
doing that is extremely small, so this is likely acceptable (more than
breaking folks who are using LLVM_ENABLE_PROJECTS=all).

Differential Revision: https://reviews.llvm.org/D132479