Inform clang when we're building for iOS
Inform clang when we're building for iOS

Apparently it needs an extra argument to tell it as such beyond just the
sysroot.

alexcrichton/gcc-rs,gabrielesvelto/gcc-rs,gabrielesvelto/gcc-rs,kosta/gcc-rs,alexcrichton/gcc-rs,kosta/gcc-rs,gabrielesvelto/gcc-rs,kosta/gcc-rs,alexcrichton/gcc-rs