__aeabi_ functions should use the AAPCS calling convention, at least with LLVM 4.0+

I'm not exactly sure about the specifics of this issue but based on 
https://github.com/rust-lang/compiler-rt/pull/26#issuecomment-259976063
I think the __aeabi aliases should use the AAPCS calling convention. `extern "aapcs" fn` seems to work. If the functions that are being aliased don't also use `extern "aapcs"` there may be some shuffling around of registers which may hamper inlining.