[compiler-rt] Pass CMake C and C++ flags to the custom libc++

When building the custom libc++ that's used by libFuzzer as well as
MSan and TSan tests, passthrough the C and C++ flags that were passed
to the compiler-rt CMake build. These may be needed to successfuly
compile the library on a particular platform.

Differential Revision: https://reviews.llvm.org/D53862

llvm-svn: 345788