Emscripten: link with -sWASM_BIGINT

When linking an executable without dynamic linking, this is a pure improvement.
It significantly reduces code size and avoids a lot of buggy behaviors. It is
supported in all browsers for many years and in all maintained versions of
Node.

It does change the ABI, so people who are dynamically linking with a library
or executable that uses the old ABI may need to turn it off. It can be disabled
if needed by passing `-Clink-arg -sWASM_BIGINT=0` to `rustc`. But few people
will want to turn it off.

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -1147,6 +1147,7 @@ impl<'a> Linker for EmLinker<'a> {
         _crate_type: CrateType,
         _out_filename: &Path,
     ) {
+        self.cmd.arg("-sWASM_BIGINT");
     }
 
     fn link_dylib_by_name(&mut self, name: &str, _verbatim: bool, _as_needed: bool) {