Wasmtime: Add support for Wasm tail calls

[fitzgen]

This adds the Config::wasm_tail_call method and --wasm-features tail-call CLI flag to enable the Wasm tail calls proposal in Wasmtime.

This PR is mostly just plumbing and enabling tests, since all the prerequisite work (Wasmtime trampoline overhauls and Cranelift tail calls) was completed in earlier pull requests.

When Wasm tail calls are enabled, Wasm code uses the tail calling convention. The tail calling convention is known to cause a 1-7% slow down for regular code that isn't using tail calls, which is why it isn't used unconditionally. This involved shepherding Tunables through to Wasm signature construction methods. The eventual plan is for the tail calling convention to be used unconditionally, but not until the performance regression is addressed. This work is tracked in
#6759

Additionally while our x86-64, aarch64, and riscv64 backends support tail calls, the s390x backend does not support them yet. Attempts to use tail calls on s390x will return errors. Support for s390x is tracked in #6530

[alexcrichton]

Nice 👍

One thing I might recommend doing though is to push a commit with prtest:full which enables tail calls by default. That'll help weed out any lingering issues and the commit can be backed out before landing to keep it off-by-default in tree

[github-actions]

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[fitzgen]

Doing a prtest:full run with tail calls enabled by default now.

[fitzgen]

Tracked down the riscv64 failure as #6780

[fitzgen]

Looks like it is just s390x that is failing prtest:full now, which is expected as s390x doesn't support tail calls yet. Gonna remove the temp commit and enqueue this to merge!

[fitzgen]

Hm looks like there is actually a second riscv64 bug lurking here.