support more architectures

[edef1c]

We currently only support x86_64, because this library grew out of an experiment for my kernel, which also only targets x86_64.
We'd like to support a few more:

• x86
• ARM
• AArch64
• MIPS

[tbelaire]

I am somewhat interested in poking around with this for an arm target. I'm going to have to take a bit to read it first though.

[whitequark]

@tbelaire I've already implemented ARM support, however it won't support panicking until rust-lang/rust#35119 is merged.

[Amanieu]

I'm interested in adding an AArch64 port. This should be easier than ARM since it uses the standard DWARF annotations for CFI.