Fix tarbin Makefile rule

The 'tarbin' Makefile rule doesn't include qemu-system-sparc64, but
should do, now that sparc64-softmmu is in the default target list.

The rule attempts to tar up binaries that were not built if a target
list was passed to the configure script -- in which case, it will
either fail, or otherwise include binaries from previous builds.

Fix both problems once and for all by building a list of binaries to
include in the tarball, using the list of targets to be built.

Signed-off-by: Stuart Brady <sdb@zubnet.me.uk>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>