Connect helper C symbols to their static library

This commit adds a `#[link]` annotation to the block defining symbols
coming from a native static library that we build and link. This is
required by rustc to get symbols to get exported correctly when linking
wasmtime into a Rust dynamic library instead of always as an rlib.

While I was at it I went ahead and renamed the symbols now that they're
no longer in C++ and they're doing setjmp/longjmp and not much else.

Closes #3006

crates/runtime/build.rs

@@ -9,5 +9,5 @@ fn main() {
             None,
         )
         .file("src/helpers.c")
-        .compile("helpers");
+        .compile("wasmtime-helpers");
 }