Auto merge of #644 - roblabla:feature-missing-SO_PASSCRED-arm, r=alexcrichton

Add missing flags in glibc arm

Adds two missing flags that are defined everywhere except for arm-linux-gnueabi for some reason : SO_PASSCRED and SO_SNDBUFFORCE.

Also adds MAP_32BIT on x86_64-linux-android and i686-linux-android.