[runtimes] Do not set XXX_STANDALONE_BUILD for libc++/abi/unwind

The runtimes build was lying to the various runtimes builds by setting
XXX_STANDALONE_BUILD=ON when they are really not being built standalone.
Only COMPILER_RT_STANDALONE_BUILD appears to be necessary, but setting it
for the other runtimes actually breaks everything.

Differential Revision: https://reviews.llvm.org/D90005