[libc] Fix installing GPU headers

The patch in D152592 changed the logic for this. We could never check if
we were on the GPU as this was before the variable was defined so I
moved it later. Secondly, we cannot use the `LLVM_BINARY_DIR` here, and
I do not know if that works in general. The problem is that it will
isntall the headers under a normal path outside of the
`LLVM_ENABLE_RUNTIMES` build. I don't know if that's correct for the
other targets, but for the GPU I need to set it back to the
CMAKE_BINARY_DIR so it works.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D153637