libc: silence unsigned->signed warnings with ioctl

This also tweaks cdefs to make __overloadable usable outside of
FORTIFY. It had to be FORTIFY-only before we had unmarked overload
support in clang+Bionic.

Bug: https://github.com/android-ndk/ndk/issues/402
Test: Internal master builds + `mma`. `mma` in Bionic fails if the
change to ioctl is undone.

Change-Id: Ib386b1786e1dca625e6d5a18682005adc734d9c1