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
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from qtbase/7d20f3dd1065a20b40cb4689783fba05190fe317)