powerpc: Add attributes for setjmp/longjmp

The setjmp function should be declared as "returns_twice", or bad
things can happen[1]. This does not actually change generated code in
my testing.

The longjmp function should be declared as "noreturn", so that the
compiler can optimise calls to it better. This makes the generated
code a little shorter.

1: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-returns_005ftwice-function-attribute

Signed-off-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c02ce4a573f3bac907e2c70957a2d1275f910013.1567605586.git.segher@kernel.crashing.org