configure: Move clang's libcpp out into flag
configure: Only use -m32 on x86-* 32 bit systems
configure: setup triples on hf arm platforms
auto merge of #13738 : buttslol/rust/armv7-support, r=alexcrichton

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?