Wasmtime+Cranelift: strip out some dead x86-32 code. (#5226)

* Wasmtime+Cranelift: strip out some dead x86-32 code.

I was recently pointed to fastly/Viceroy#200 where it seems some folks
are trying to compile Wasmtime (via Viceroy) for Windows x86-32 and the
failures may not be loud enough. I've tried to reproduce this
cross-compiling to i686-pc-windows-gnu from Linux and hit build failures
(as expected) in several places.  Nevertheless, while trying to discern
what others may be attempting, I noticed some dead x86-32-specific code
in our repo, and figured it would be a good idea to clean this up.
Otherwise, it (i) sends some mixed messages -- "hey look, this codebase
does support x86-32" -- and (ii) keeps untested code around, which is
generally not great.

This PR removes x86-32-specific cases in traphandlers and unwind code,
and Cranelift's native feature detection. It adds helpful compile-error
messages in a few cases. If we ever support x86-32 (contributors
welcome! The big missing piece is Cranelift support; see #1980), these
compile errors and git history should be enough to recover any knowledge
we are now encoding in the source.

I left the x86-32 support in `wasmtime-fiber` alone because that seems
like a bit of a special case -- foundation library, separate from the
rest of Wasmtime, with specific care to provide a (presumably working)
full 32-bit version.

* Remove some extraneous compile_error!s, already covered by others.

cranelift/native/src/lib.rs

@@ -46,7 +46,12 @@ pub fn builder_with_options(infer_native_flags: bool) -> Result<isa::Builder, &'
         isa::LookupError::Unsupported => "unsupported architecture",
     })?;
 
-    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+    #[cfg(target_arch = "x86")]
+    {
+        compile_error!("Unsupported architecture");
+    }
+
+    #[cfg(target_arch = "x86_64")]
     {
         use cranelift_codegen::settings::Configurable;