rewrite default spec handling

instead of symlinking (on unix) or creating a forwarding spec (on
windows), just put the default specs into (the bootstrapped)
QLibraryInfo.

Change-Id: I595500ef7399f77cb8ec117c4303bc0a2ffe505f
(cherry picked from qttools/0e75115e557b4cf1c737e3e5524f876a6b85a39e)
(cherry picked from qttools/769fa282ac8a4b98698dada6969452363e0eb415)
Reviewed-by: Daniel Teske <daniel.teske@digia.com>