Bug 1516214: Enable accessibility on aarch64 Windows. r=glandium

Accessibility now compiles and works, so we can re-enable it!
This effectively backs out the moz.configure part of 02cd44e39566.

Differential Revision: https://phabricator.services.mozilla.com/D19650

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

toolkit/moz.configure

@@ -1162,15 +1162,7 @@ set_config('MIDL_FLAGS', midl_flags)
 # Accessibility
 # ==============================================================
 
-@depends(target)
-def a11y_default(target):
-    # Accessibility doesn't work on aarch64-windows yet.
-    if target.kernel == 'WINNT' and target.cpu == 'aarch64':
-        return False
-    return True
-
-option('--enable-accessibility',
-       default=a11y_default, help='{Enable|Disable} accessibility support')
+option('--disable-accessibility', help='Disable accessibility support')
 
 @depends('--enable-accessibility', check_for_midl, midl, c_compiler)
 def accessibility(value, check_for_midl, midl, c_compiler):