[libc] Small improvements to libc cmake

This patch includes:
1. Better error message when cmake finds incompatible triple
2. Added missing header dependencies libc/include/CMakeLists.txt as per
   app.td
3. Removed unused $LLVM_LIBC_INCLUDE_DIRS cmake variable

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D145496