powerpc: Wire up sys_bpf() syscall

This patch wires up the new syscall sys_bpf() on powerpc.

Passes the tests in samples/bpf:

    #0 add+sub+mul OK
    #1 unreachable OK
    #2 unreachable2 OK
    #3 out of range jump OK
    #4 out of range jump2 OK
    #5 test1 ld_imm64 OK
    #6 test2 ld_imm64 OK
    #7 test3 ld_imm64 OK
    #8 test4 ld_imm64 OK
    #9 test5 ld_imm64 OK
    #10 no bpf_exit OK
    #11 loop (back-edge) OK
    #12 loop2 (back-edge) OK
    #13 conditional loop OK
    #14 read uninitialized register OK
    #15 read invalid register OK
    #16 program doesn't init R0 before exit OK
    #17 stack out of bounds OK
    #18 invalid call insn1 OK
    #19 invalid call insn2 OK
    #20 invalid function call OK
    #21 uninitialized stack1 OK
    #22 uninitialized stack2 OK
    #23 check valid spill/fill OK
    #24 check corrupted spill/fill OK
    #25 invalid src register in STX OK
    #26 invalid dst register in STX OK
    #27 invalid dst register in ST OK
    #28 invalid src register in LDX OK
    #29 invalid dst register in LDX OK
    #30 junk insn OK
    #31 junk insn2 OK
    #32 junk insn3 OK
    #33 junk insn4 OK
    #34 junk insn5 OK
    #35 misaligned read from stack OK
    #36 invalid map_fd for function call OK
    #37 don't check return value before access OK
    #38 access memory with incorrect alignment OK
    #39 sometimes access memory with incorrect alignment OK
    #40 jump test 1 OK
    #41 jump test 2 OK
    #42 jump test 3 OK
    #43 jump test 4 OK

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
[mpe: test using samples/bpf]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>