Add platform 0.4.21: Linux 32-bit ARM with bionic ("Android CLI")
Fallback defines for S_IREAD/S_IWRITE

The glibc documentation describes S_IREAD/S_IWRITE as "obsolete
synonym[s] provided for BSD compatibility." Bionic (the libc used on
Android) does not define S_IREAD/S_IWRITE at all.

This fix was originally submitted by @giuliolunati.