[build] Fix misaligned table entries when using gcc 4.5

[build] Fix misaligned table entries when using gcc 4.5

Declarations without the accompanying __table_entry cause misalignment
of the table entries when using gcc 4.5.  Fix by adding the
appropriate __table_entry macro or (where possible) by removing
unnecessary forward declarations.

Signed-off-by: Piotr Jaroszyński <p.jaroszynski@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>