[runtimes] Remove support for standalone builds

[runtimes] Remove support for standalone builds

Standalone build have been deprecated for some time now, so this
commit removes support for those builds entirely from libc++, libc++abi
and libunwind.

This, along with the removal of other legacy ways to build, will allow
for major build system simplifications.

Differential Revision: https://reviews.llvm.org/D119255