[CMake] Convert paths to the right form in standalone builds on Windows

The paths output from llvm-config --cmakedir and from clang
--print-libgcc-file-name can contain backslashes, while CMake
can't handle the paths in this form.

This matches what compiler-rt already does (since SVN r203789
and r293195).

Differential Revision: https://reviews.llvm.org/D48355

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@335171 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed