powerpc: Move and rename func_descr_t

There are three architectures with function descriptors, try to
have common names for the address they contain in order to
refactor some functions into generic functions later.

powerpc has 'entry'
ia64 has 'ip'
parisc has 'addr'

Vote for 'addr' and update 'func_descr_t' accordingly.

Move it in asm/elf.h to have it at the same place on all
three architectures, remove the typedef which hides its real
type, and change it to a smoother name 'struct func_desc'.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/529b2ba1d001e8f628ef0d30e8044c9b3d0a4921.1644928018.git.christophe.leroy@csgroup.eu