[CMake][compiler-rt] Replace Windows backslashes with CMake ones

XRay builds uses llvm-config to obtain the ldflags and libs and then
passes those to CMake. Unfortunately, this breaks on Windows because
CMake tries to interpret backslashes followed by certain characters
as flags. We need to rewrite these into forward slashes that are used
by CMake (even on Windows).

Differential Revision: https://reviews.llvm.org/D73523