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/src/traphandlers/macos.rs

@@ -33,7 +33,7 @@
 
 #![allow(non_snake_case)]
 
-use crate::traphandlers::{tls, Trap, Unwind};
+use crate::traphandlers::{tls, wasmtime_longjmp, Trap};
 use mach::exception_types::*;
 use mach::kern_return::*;
 use mach::mach_init::*;
@@ -389,7 +389,7 @@ unsafe extern "C" fn unwind(wasm_pc: *const u8) -> ! {
         state.jmp_buf.get()
     });
     debug_assert!(!jmp_buf.is_null());
-    Unwind(jmp_buf);
+    wasmtime_longjmp(jmp_buf);
 }
 
 thread_local! {

crates/runtime/src/traphandlers/unix.rs

@@ -1,4 +1,4 @@
-use crate::traphandlers::{tls, Trap, Unwind};
+use crate::traphandlers::{tls, wasmtime_longjmp, Trap};
 use std::cell::RefCell;
 use std::convert::TryInto;
 use std::io;
@@ -99,7 +99,7 @@ unsafe extern "C" fn trap_handler(
             return true;
         }
         info.capture_backtrace(pc);
-        Unwind(jmp_buf)
+        wasmtime_longjmp(jmp_buf)
     });
 
     if handled {

crates/runtime/src/traphandlers/windows.rs

@@ -1,4 +1,4 @@
-use crate::traphandlers::{tls, Trap, Unwind};
+use crate::traphandlers::{tls, wasmtime_longjmp, Trap};
 use std::io;
 use winapi::um::errhandlingapi::*;
 use winapi::um::minwinbase::*;
@@ -69,7 +69,7 @@ unsafe extern "system" fn exception_handler(exception_info: PEXCEPTION_POINTERS)
             EXCEPTION_CONTINUE_EXECUTION
         } else {
             info.capture_backtrace(ip);
-            Unwind(jmp_buf)
+            wasmtime_longjmp(jmp_buf)
         }
     })
 }