This patch fixes a couple of bugs introduced in CL 210778 and CL 207606:

- apply the same version selection scheme in loader.CreateExtSym that
  we're currently using for loader.Create (since the two functions
  will be used in the same way by the host object loader)

- add code to the loader's NewLoader function to create initial map
  values for some of the map-based symbol attributes (somewhere along
  the line the code to do this seems to have gotten lost, so this
  patch adds it back).

- fix a coding error in growAttrBitmaps (wrong bitmap passed to
  append when extending attrOnList)

Change-Id: Ie0c8c6876428bb21d788c19a7a2db945ac649fac
Reviewed-on: https://go-review.googlesource.com/c/go/+/212097


Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>