Re-enable iOS on CI

[alexcrichton]

This started failing with a recent xcode update from Travis and had to be disabled in #1051 due to errors like this -- https://travis-ci.org/rust-lang/libc/jobs/410547612

Looks like the simulated program is no longer printing out anything :(

[gnzlbg]

We are seeing these same errors in mach and packed_simd fitzgen/mach#39, rust-lang/packed_simd#26

cc @kali

[gnzlbg]

The i386-apple-ios target is deprecated on xcode10. I propose to revert the xcode version of that target to xcode9.2, which should work without issues. For some reason, no output is emitted with xcode9.4, so we should leave this issue open until somebody fixes that.

[gnzlbg]

With xcode10, rust-lang/rust#55477, x86_64-apple-ios and i386-apple-ios actually fail to link, so libc does not compile anymore for these targets apparently.