Update fibers to avoid no-return functions (#12928)

* Update fibers to avoid no-return functions

This commit is aimed at fixing the ASAN false positives in #12899.
Initially the fix there was to in

Update fibers to avoid no-return functions (#12928)

* Update fibers to avoid no-return functions

This commit is aimed at fixing the ASAN false positives in #12899.
Initially the fix there was to invoke some `__asan_*` intrinsics, and
I ended up finding a sort of smaller set of `__asan_*` intrinsics to
call as well. In the end what's happening though is that fibers, upon
terminating, have a few frames of Rust code on the stack before
switching off. To ASAN these frames never returned so when a stack is
subsequently reused ASAN is tricked into thinking this is buffer
overflow or use-after-free since it's stomping on frames that haven't
returned.

The fix in this commit is to avoid this style of function which doesn't
returns. Functions which don't return in Rust are easy to leak memory
from and are a hazard from a safety perspective as well (e.g. it's
unsafe to skip running destructors of stack variables). I feel we've had
better success over time with "all Rust functions always return" and so
what's what was applied here. Unlike #12899 or my thoughts on that PR
this does not have any new `__asan_*` intrinsic calls. Instead what this
does is it shuffles around responsibility for what exact piece of the
infrastructure is responsible for what. Specifically `fiber_start`
functions now actually return, meaning the `wasmtime_fiber_start` naked
function actually resumes execution, unlike before. The
`wasmtime_fiber_start` then delegates to `wasmtime_fiber_switch`
immediately to perform the final switch.

Effectively there's now only two function frames that never return, and
both of these frames are handwritten inline assembly. This means that
ASAN gets to see that all normal functions return and updates all of its
metadata accordingly. The end result is that the original issue from #12899
is fixed and this I feel is in general more robust as well.

One caveat is that the handwritten `wasmtime_fiber_start` assembly needs
to invoke a sibling `wasmtime_fiber_switch_` function. In lieu of trying
to figure out how to get PIC-vs-not calls working (e.g. static calls)
I've opted to use indirect function calls and pointers instead. This
mirrors historical changes in our fiber implementation too.

* Fix CI builds

* Fix miri

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Add fiber implementation for riscv32imac (#12506)

* Add fiber implementation for riscv32imac

* Adjusted formatting around commas

* Added padding infront of `last_sp`