This sets a precedence in upstream for merging architecture agnostic
C bindings into one block, rather than separate duplicated blocks.

A nice side-effect is it almost completes the C bindings for
s390-linux-gnu and sparc-linux-gnu, and fixes a bug on MIPS32 where
O_SYNC had the wrong value.

Backported from upstream druntime master.

Reviewed-on: https://github.com/dlang/druntime/pull/2357

From-SVN: r266593