Fix android build

getpwent is not available on android until API level 21, and even then
it is only available when doing a non-static link. Since android's
concept of users is very different from linux, it's doubtful the home
directory resolution would be useful, so I approximate this state by
just not using getpwent on android.

We've had another getpwent occurance in FileSpec for a while -- it
wasn't causing problems because it was stripped out by the linker, but I
disable that also, for consistency's sake.

llvm-svn: 297612