Add TargetIsa::map_dwarf_register; fixes #1471

[abrown]

This exposes the functionality of fde::map_reg on the TargetIsa trait, avoiding compilation errors on architectures where register mapping is not yet supported. The change is conditially compiled under the unwind feature.

[abrown]

@peterhuene, not sure how this will play with #1466 but this does seem like the better long-term solution. Also, I hope my liberal use of #[cfg(feature = "unwind")] is what we are looking for here.

[abrown]

@stefson, any chance you can try a compile of this branch to see if it actually fixes #1471?

[peterhuene]

LGTM 👍, but let's get @yurydelendik's opinion as well.

Just one nit comment; I think this should unblock the arm build of wasmtime-environ.

[peterhuene]

I'll tackle any conflicts this might cause for #1466.

[yurydelendik]

LGTM 👍, but let's get @yurydelendik's opinion as well.

I'm good with the approach.

[github-actions]

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[stefson]

@stefson, any chance you can try a compile of this branch to see if it actually fixes #1471?

@abrown it seems to depend on an unreleased version of witx ?

/tmp/wasmtime $ cargo build --target armv7-unknown-linux-gnueabihf
Updating crates.io index
error: failed to select a version for the requirement witx = "^0.8.5"
candidate versions found which didn't match: 0.8.4
location searched: /tmp/wasmtime/crates/wasi-common/WASI/tools/witx
required by package wig v0.15.0 (/tmp/wasmtime/crates/wasi-common/wig)

[abrown]

@stefson, I see witx at 0.8.5 available on crates.io so I'm not exactly sure what is going on (@pchickey, is it clear to you?). Perhaps there is a git submodule update needed when jumping on to this branch?

[stefson]

I forgot to update the submodules, I'm sorry :c

But yes, with your branch it is now back to the old state of an error in wasmtime-obj, where it flakes out with an unsupported platform error, which is expected.