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/helpers.c

@@ -14,7 +14,7 @@
 #define platform_jmp_buf sigjmp_buf
 #endif
 
-int RegisterSetjmp(
+int wasmtime_setjmp(
     void **buf_storage,
     void (*body)(void*, void*),
     void *payload,
@@ -28,7 +28,7 @@ int RegisterSetjmp(
   return 1;
 }
 
-void Unwind(void *JmpBuf) {
+void wasmtime_longjmp(void *JmpBuf) {
   platform_jmp_buf *buf = (platform_jmp_buf*) JmpBuf;
   platform_longjmp(*buf, 1);
 }