boost: fix sparc64 compile, set the correct instruction-set

The default for sparc is v7, which doesn't work for sparc64.
Use ultrasparc for sparc64.
Fixes following autobuild errors:
http://autobuild.buildroot.net/results/0cad0a6eddc153098bb6d1501afd1cf4a57a82d1/



Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>