[libc][bazel] split support_standalone_cpp target

previously the support_standalone_cpp target contained all of the files
in the __support/cpp folder. This change splits these out so that only
what is needed is included. In addition, this change adds the new
support files that previously didn't have targets.

Reviewed By: lntue, gchatelet

Differential Revision: https://reviews.llvm.org/D121314