ppc: Fix rfi/rfid/hrfi/... emulation

This reworks emulation of the various "rfi" variants. I removed
some masking bits that I couldn't make sense of, the only bit that
I am aware we should mask here is POW, the CPU's MSR mask should
take care of the rest.

This also fixes some problems when running 32-bit userspace under
a 64-bit kernel.

This patch broke 32bit OpenBIOS when run under a 970 cpu. A fix was
proposed here :

    https://www.coreboot.org/pipermail/openbios/2016-June/009452.html

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
[clg: updated the commit log with the reference of the openbios fix ]
Signed-off-by: Cédric Le Goater <clg@kaod.org>
[dwg: Remove hunk which disabled rfi on 64-bit CPUS.  The change was
 correct, but we need to fix OpenBIOS before applying it]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

target-ppc/translate.c

@@ -4119,6 +4119,10 @@ static void gen_rfi(DisasContext *ctx)
 #if defined(CONFIG_USER_ONLY)
     gen_inval_exception(ctx, POWERPC_EXCP_PRIV_OPC);
 #else
+    /* FIXME: This instruction doesn't exist anymore on 64-bit server
+     * processors compliant with arch 2.x, we should remove it there,
+     * but we need to fix OpenBIOS not to use it on 970 first
+     */
     /* Restore CPU state */
     if (unlikely(ctx->pr)) {
         gen_inval_exception(ctx, POWERPC_EXCP_PRIV_OPC);