sparc64: fix fault handling in NGbzero.S and GENbzero.S

sparc64: fix fault handling in NGbzero.S and GENbzero.S

When any of the functions contained in NGbzero.S and GENbzero.S
are being run, we may end up taking a fault when executing one
of the store alternate address space instructions. If this
happens, the exception handler does not restore the %asi
register.

This commit fixes the issue by introducing a new exception
handler that ensures the %asi register is restored when
a fault is handled.

Orabug: 25577560

Signed-off-by: Dave Aldridge <david.j.aldridge@oracle.com>
Reviewed-by: Rob Gardner <rob.gardner@oracle.com>
Reviewed-by: Babu Moger <babu.moger@oracle.com>
Signed-off-by: Allen Pais <allen.pais@oracle.com>