objtool/powerpc: Add support for decoding all types of uncond branches

Add support for 'bla' instruction.

This is done by 'flagging' the address as an absolute address so that
arch_jump_destination() can calculate it as expected. Because code is
_always_ 4 bytes aligned, use bit 30 as flag.

Also add support for 'b' and 'ba' instructions. Objtool call them jumps.

And make sure the special 'bl .+4' used by clang in relocatable code is
not seen as an 'unannotated intra-function call'. clang should use the
special 'bcl 20,31,.+4' form like gcc but for the time being it does not
so lets work around that.

Link: https://github.com/llvm/llvm-project/issues/128644
Reviewed-by: Segher Boessenkool <segher@kewrnel.crashing.org>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/bf0b4d554547bc34fa3d1af5b4e62a84c0bc182b.1740470510.git.christophe.leroy@csgroup.eu

tools/objtool/arch/powerpc/decode.c

@@ -55,12 +55,17 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 
 	switch (opcode) {
 	case 18: /* b[l][a] */
-		if ((ins & 3) == 1) /* bl */
+		if (ins == 0x48000005)	/* bl .+4 */
+			typ = INSN_OTHER;
+		else if (ins & 1)	/* bl[a] */
 			typ = INSN_CALL;
+		else		/* b[a] */
+			typ = INSN_JUMP_UNCONDITIONAL;
 
 		imm = ins & 0x3fffffc;
 		if (imm & 0x2000000)
 			imm -= 0x4000000;
+		imm |= ins & 2;	/* AA flag */
 		break;
 	}
 
@@ -77,6 +82,9 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 
 unsigned long arch_jump_destination(struct instruction *insn)
 {
+	if (insn->immediate & 2)
+		return insn->immediate & ~2;
+
 	return insn->offset + insn->immediate;
 }