DwarfFile: Add missing continue for skip case.
Update Vision x86{_gcc2} packages to r949.

Updated to no longer rely on deprecated get_system_info() call.
Add support for explicitly preventing rebuilding packages

Due to depending on dynamically built files, package files will always
be rebuilt when they are needed (e.g. when an image is built). The build
variable HAIKU_DONT_REBUILD_PACKAGES can be defined to prevent
rebuilding existing package files (even ones that are out-of-date).

Main target for this change is buildbot. We want to make sure that the
packages in the repository it builds are exactly identical to the ones
in the images, which may not be the case when the packages are rebuilt
(due to different timestamps of contained files). The respective build
order should be:

1. Build repository.
2. Without cleaning the generated directory, build different image types
   with HAIKU_DONT_REBUILD_PACKAGES defined.