x86-64: use PC-relative exception table entries

x86-64: use PC-relative exception table entries

... thus allowing to make the entries half their current size. Rather
than adjusting all instances to the new layout, abstract the
construction the table entries via a macro (paralleling a similar one
in recent Linux).

Also change the name of the section (to allow easier detection of
missed cases) and merge the final resulting output sections into
.data.read_mostly.

Signed-off-by: Jan Beulich <jbeulich@novell.com>