[libc++] Provide 'buildhost=<platform> feature for the tests.

The target platform could differ from the host platform for the cross
platform builds. Some tests are depended on the build host features and
they need to determine a proper platform environment.

This commit adds a build host platform name feature for the libc++ tests
in format `buildhost=<platform>`, such as `buildhost=linux`, `buildhost=darwin`,
`buildhost=windows`, etc.

The Windows host gets two features: one `buildhost=windows` and another based
on Windows "sub-system", such as `buildhost=win32`, `buildhost=cygwin`, etc.

Differential Revision: https://reviews.llvm.org/D102045