Remove some cross-compilation kludges

I don't think these should be necessary. If something breaks as
a result then we can look at why.
Use a pthread-based implementation of Itimer.c on iOS

Patch from Stephen Blackheath.

timer_create doesn't exist and setitimer doesn't fire on iOS, so we're
using a pthreads-based implementation. It may be to do with
interference with the signals of the debugger. Revisit. See #7723.
pass the correct -Dxxx_HOST_ARCH and -Dxxx_HOST_OS to hsc2hs; Fixes #7761.

Patch from Stephen Blackheath.