allow setting codegen-backend per-target

[flippette]

Problem

i'm currently testing the codegen-backend option, but every target besides x86_64-unknown-linux-gnu does not work with cranelift. i would like to be able to select the backend for each individual target, or even better, for each cfg() target.

Proposed Solution

add a codegen-backend field to [target.<triple>] and [target.<cfg-expr>] sections.

Notes

p.s. would also be nice if we could somehow do something akin to

[target.x86_64-unknown-linux-gnu.profile.dev]
codegen-backend = "cranelift"

[weihanglo]

I consider this as a subset of #4897, so close in favor of that.

In the mean time, you can use "custom profile" instead. A bit more typing but should work.

Feel free to add more comments here or there if you have any further question :)