Log Message:
go: Improve PRINT_PLIST_AWK patterns and avoid possible double definition

- Move GO_PLATFORM definition in lang/go/version.mk in order that also lang/go*
  packages can (re)use it
- Change PRINT_PLIST_AWK pattern that replace all ${GO_PLATFORM} and apply it
  only when ${GO_PLATFORM} is a directory (between two "/").  There \ 
are only
  3 exceptions to that in lang/go14.
  Move it to version.mk so lang/go* PLIST can be mostly automatically
  generated.

These changes should help to avoid (most) manual editing of
lang/go*/PLIST.

Discussed with and thanks to <bsiegert>!