powerpc/bpf/64: Add instructions for atomic_[cmp]xchg

This adds two atomic opcodes BPF_XCHG and BPF_CMPXCHG on ppc64, both
of which include the BPF_FETCH flag.  The kernel's atomic_cmpxchg
operation fundamentally has 3 operands, but we only have two register
fields. Therefore the operand we compare against (the kernel's API
calls it 'old') is hard-coded to be BPF_REG_R0. Also, kernel's
atomic_cmpxchg returns the previous value at dst_reg + off. JIT the
same for BPF too with return value put in BPF_REG_0.

  BPF_REG_R0 = atomic_cmpxchg(dst_reg + off, BPF_REG_R0, src_reg);

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Tested-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com> (ppc64le)
Reviewed-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220610155552.25892-4-hbathini@linux.ibm.com