bug 1598286: toolkit: enable remote agent on Windows AArch64; r=remote-protocol-reviewers,whimboo,froydnj

The remote agent used to not compile on Windows AArch64 due to iovec's
dependency on a version of winapi without support for this architecture.

Now that the remote agent has upgraded to http 0.2, which depends on
a version of the bytes crate that has moved away from iovec in favour
of std::io::IoSlice, we are able to turn on support for Windows AArch64.

This in turn will also fix bug 1606935 because the browser-chrome
test manifest for M(remote) will no longer be empty.  It was a
regression caused by 1603930 where we fixed a logic error causnig
ENABLE_REMOTE_AGENT to be inappropriately set on non-trunk branches.

Differential Revision: https://phabricator.services.mozilla.com/D58767

--HG--
extra : moz-landing-system : lando

toolkit/moz.configure

@@ -935,11 +935,6 @@ def remote_default(target, project, milestone):
     if target.os == 'Android':
         return False
 
-    # winapi 0.2.8 does not work on AArch64
-    # TODO(ato): https://bugzil.la/1598286
-    if target.os == 'WINNT' and target.cpu == 'aarch64':
-        return False
-
     # enable on Firefox Nightly channel only
     # TODO(ato): https://bugzil.la/1544393
     return project == 'browser' and milestone.is_nightly