[SPARC64]: Mark __ex_table section correctly.

[SPARC64]: Mark __ex_table section correctly.

We must use the "a" (allocate) attribute every time we
emit an entry into the __ex_table section.

For consistency, use "a" instead of #alloc which is some
Solaris compat cruft GNU as provides on Sparc.

Signed-off-by: David S. Miller <davem@davemloft.net>