Pull the tier-2 card and change the sparc64 ZFS loader to no longer probe
all diskN aliases for providers (which more or less corresponds to how the
x86 version behaves) but instead probe only those listed in the boot-device
OFW environment variable. This has the following advantages:

• avoids otherwise unavoidable OFW warnings about failures to open disks for which aliases exist but no actual hardware is connected
• avoids issues due to different diskN naming schemes
• aligns us with Solaris

MFC after: 3 days