objtool/powerpc: Enable objtool to be built on ppc

This patch adds [stub] implementations for required functions, inorder
to enable objtool build on powerpc.

[Christophe Leroy: powerpc: Add missing asm/asm.h for objtool,
Use local variables for type and imm in arch_decode_instruction(),
Adapt len for prefixed instructions.]

Tested-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Reviewed-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Sathvika Vasireddy <sv@linux.ibm.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221114175754.1131267-16-sv@linux.ibm.com