perl: build with perlcross instead of qemu

The perlcross project makes it possible to properly cross-compile
perl.  It creates a host-miniperl that is configured for the target
and uses that to cross-compile the perl modules.

Unfortunately there are still a few hacks needed to make it work.
Proper fixes can be developed and upstreamed later.

Since there is no longer a dependency on qemu, it works on all
architectures again.

Also removed some config options:
- BR2_PACKAGE_PERL_CUSTOM_INSTALL just allows a selection of modules;
this can also be encoded by an empty BR2_PACKAGE_PERL_MODULES.
- BR2_PACKAGE_PERL_DB_FILE and BR2_PACKAGE_PERL_GDBM_FILE can be
derived automatically from the package configs.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>