Support armv7l-hf

This was required to get ./configure to work on my armv7 test machine.

I haven't found anything sane to feature gate `hf` on that's pokable from the context of the configure script.

It also seems that gcc doesn't work on armv7 by default (rust wants to pass it `-m32` which isn't supported), would it be preferential to make the default `--enable-clang` on arm, or remove the `-m32` flag on that platform?