Merge #42

42: Only checking for necessary extensions when linking r=Disasm a=ilya-epifanov

`M`, `A`, `F` and `D` (as part of `G`) extensions don't matter at all, so generating binaries for them is not necessary, really. Also, if another rust RISC-V target were to come up, this crate would not support it.

In this PR I'm restricting a list of extensions to consider when generating static archives and linking to `I` and `C`.

Co-authored-by: Ilya Epifanov <elijah.epifanov@gmail.com>