std: Add compatibility with android-9

std: Add compatibility with android-9

The Gecko folks currently use Android API level 9 for their builds, so they're
requesting that we move back our minimum supported API level from 18 to 9. Turns
out, ABI-wise at least, there's not that many changes we need to take care of.
The `ftruncate64` API appeared in android-12 and the `log2` and `log2f` APIs
appeared in android-18. We can have a simple shim for `ftruncate64` which falls
back on `ftruncate` and the `log2` function can be approximated with just
`ln(f) / ln(2)`.

This should at least get the standard library building on API level 9, although
the tests aren't quite happening there just yet. As we seem to be growing a
number of Android compatibility shims, they're now centralized in a common
`sys::android` module.