[gccsdk] paths.h

Hi

The attached patch adds the paths.h header. This header is common on Unix systems, and contains defines for common system paths. While the contents don't appear to be standardised, the patch adds a reduced set of defines from other platforms that are relevant to UnixLib.

Sources:

https://docs.oracle.com/cd/E36784_01/html/E36873/paths.h-3head.html

https://code.woboq.org/userspace/glibc/sysdeps/unix/sysv/linux/paths.h.html  

https://chromium.googlesource.com/native_client/nacl-newlib/+/master/newlib/libc/include/paths.h

Regards

Cameron