tools: updates for minimum Go version 1.6

Details:
- remove go1.5 "default version" labels on most files
- remove go1.6 labels on a few files
- go/loader: use conf.Cwd not "." in a couple places; update faulty
  test expectations.  (Not sure why this wasn't caught sooner.)
- go/ssa/interp: add 'mono' result to time.now intrinsic
- go/gcimporter15/bimport.go: make consistent with the version in gc
- go/ssa/interp: update test error message
- go/ssa: update a comment

The go/gcimporter15/bexport.go logic is stale and needs to be brought
up to date.  Needs a separate CL since it's tricky.

Tested on go1.6, go1.7, go1.8.

Change-Id: I841189d30e131b7c49a4e8690ea7c40b55041bae
Reviewed-on: https://go-review.googlesource.com/36540
Reviewed-by: Robert Griesemer <gri@golang.org>
69 files changed