Fix cc.h header to run with C++ code

We need to add the extern "C" {...} guard around `cmsis_compiler.h`
header because of a known issue with cmsis_gcc.h file
https://github.com/ARM-software/CMSIS_5/issues/617

 that causes
an error during C++ compiling

In cc.h file there is also declaration of `rand()` function.
The `stdlib.h` already contains this declaration so we have a conflict
during C++ compiling. Change it to include `stdlib.h` directly.

Changes allow for building LwIP components in C++ projects.

Signed-off-by: Artur Tynecki <Artur.Tynecki@arm.com>