target/ppc: extend eieio for POWER9

target/ppc: extend eieio for POWER9

POWER9 introduced a new variant of the eieio instruction using bit 6
as a hint to tell the CPU it is a store-forwarding barrier.

The usage of this eieio extension was recently added in Linux 4.17
which activated the "support for a store forwarding barrier at kernel
entry/exit".

Unfortunately, it is not possible to insert this new eieio instruction
without considerable change in ppc_tr_translate_insn(). So instead we
loosen the QEMU eieio instruction mask and modify the gen_eieio()
helper to test for bit6. On non-POWER9 CPUs, the bit6 is just ignored
but a warning is emitted as this is not an instruction software should
be using.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>