Implement Extended Berkeley Packet Filter on Powerpc 32

Test result with test_bpf module:

	test_bpf: Summary: 378 PASSED, 0 FAILED, [354/366 JIT'ed]

Registers mapping:

	[BPF_REG_0] = r11-r12
	/* function arguments */
	[BPF_REG_1] = r3-r4
	[BPF_REG_2] = r5-r6
	[BPF_REG_3] = r7-r8
	[BPF_REG_4] = r9-r10
	[BPF_REG_5] = r21-r22 (Args 9 and 10 come in via the stack)
	/* non volatile registers */
	[BPF_REG_6] = r23-r24
	[BPF_REG_7] = r25-r26
	[BPF_REG_8] = r27-r28
	[BPF_REG_9] = r29-r30
	/* frame pointer aka BPF_REG_10 */
	[BPF_REG_FP] = r17-r18
	/* eBPF jit internal registers */
	[BPF_REG_AX] = r19-r20
	[TMP_REG] = r31

As PPC32 doesn't have a redzone in the stack, a stack frame must always
be set in order to host at least the tail count counter.

The stack frame remains for tail calls, it is set by the first callee
and freed by the last callee.

r0 is used as temporary register as much as possible. It is referenced
directly in the code in order to avoid misusing it, because some
instructions interpret it as value 0 instead of register r0
(ex: addi, addis, stw, lwz, ...)

The following operations are not implemented:

		case BPF_ALU64 | BPF_DIV | BPF_X: /* dst /= src */
		case BPF_ALU64 | BPF_MOD | BPF_X: /* dst %= src */
		case BPF_STX | BPF_XADD | BPF_DW: /* *(u64 *)(dst + off) += src */

The following operations are only implemented for power of two constants:

		case BPF_ALU64 | BPF_MOD | BPF_K: /* dst %= imm */
		case BPF_ALU64 | BPF_DIV | BPF_K: /* dst /= imm */
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/61d8b149176ddf99e7d5cef0b6dc1598583ca202.1616430991.git.christophe.leroy@csgroup.eu