First Android/x86 patch that produces a working D executable

This is the first patch where I was able to build actual D Android/x86 executables that use the core of druntime, cleaned up a bit since.  I tried to follow the existing naming conventions in druntime, like sigaction_t and stat_t.  There were two unnecessary uses of version(Posix) that I removed.  FD_(CLR|SET|ISSET|ZERO) are defined in assembly in bionic, so I didn't bother translating them.  This patch alone can't actually be used, as it needs a sections_android.d and mak/SRCS needs to be trimmed to only include these files, but those patches will come next.