Reland "Passthrough additional flags to custom libcxx CMake build"

This is needed when we're cross-compiling compiler-rt.

Differential Revision: https://reviews.llvm.org/D47834

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk/lib/fuzzer@334570 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed