powerpc/Kconfig: Select FUNCTION_ALIGNMENT_4B

Commit d49a0626 ("arch: Introduce CONFIG_FUNCTION_ALIGNMENT")
introduced a generic function-alignment infrastructure. Move to using
FUNCTION_ALIGNMENT_4B on powerpc, to use the same alignment as that of
the existing _GLOBAL macro.

Signed-off-by: Sathvika Vasireddy <sv@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/21892186ec44abe24df0daf64f577dac0e78783f.1702045299.git.naveen@kernel.org