[libc] Set default CXX_STANDARD to C++17 and let targets set their own standard if needed.

CMAKE_CXX_STANDARD 14 is set in the llvm-project/llvm folder overriding all COMPILE_OPTIONS -std=c++17.  We need to override the CXX_STANDARD property of the target in order to set the correct C++ standard flags.

Reviewed By: gchatelet

Differential Revision: https://reviews.llvm.org/D118871