Add ~sparc keyword. The -1.5-r1 version should be the new stable version

Add ~sparc keyword.  The -1.5-r1 version should be the new stable version
(upgrade from current stable, which is 1.5_rc5), but it can't start as stable
because it currently has no sparc keyword of any kind.  But both -r1 and -r2
build just fine.  Notice Bug #89510 for sparc comments.
Package-Manager: portage-2.1.2_rc2-r2