Implement basic inline asm support (#72)

* Implement basic support for inline assembly

* Disable LTO

We don't support it yet at all

* Handle `inout(reg) var` correctly

Turns out that `+` readwrite output registers cannot be tied with
input variables.

* Add limited support for llvm_asm!

* Handle CHANNEL correctly

* Add support for arbitrary explicit registers

* Handle symbols properly

* Add rudimentary asm tests

* Exclude llvm_asm! tests from tests runs

* Insert `__builtin_unreachable()` after diverging asm blocks