[Downstream Change] Include newlib.h to define _NEWLIB_VERSION #378
Description
What is the change
This change adds check and include of newlib.h next to the same for picolibc.h to define _NEWLIB_VERSION.
Why this change cannot be done upstream
This was discussed in libc++ monthly sync on 10-06-2025 https://docs.google.com/document/d/1yC-3bE6pefVizUhfiUs1Shv11EzuL0KOklGSLqto6dk/edit?tab=t.0 and the conclusion was that it is a bad idea to include "random" header files, we need to make use of the C library definition in the CMake (now only LLVM libc is supported) to get C libarry specific settings.
This will take quite some time to design and implement, thus a downstream change is made to fix C++ library support with newlib.