x86: move declaration of the exception_table to C

x86: move declaration of the exception_table to C

This makes the code cleaner because there's no need to declare the
exception_table in assembly, and also fixes the following error when
using clang's integrated assembler:

entry.S:834:15: error: unexpected token in '.rept' directive
        .rept 32 - ((. - exception_table) / 8)
              ^
entry.S:836:14: error: unmatched '.endr' directive
        .endr
             ^

This should be a non-functional change.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>