First set of stdc changes for Android/x86

You cannot actually link against core.stdc in druntime using these patches, as a few missing symbols remain, but since most of it is just declarations for symbols in libc, you should be able to use this patch as D "headers" to write basic command-line utilities for Android/x86.  The few symbols that would be provided by core.stdc, such as stderr/stdin/stdout on Android, wouldn't work in that scenario.