ppc: Fix catching some segfaults in user mode

The usermode "translate" code generates an error code value that
has the "is_write" bit set, which causes our switch/case to miss
and display "Invalid segfault errno" and a spurrious second state
dump. Fix it.
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

linux-user/main.c

@@ -1656,6 +1656,7 @@ void cpu_loop(CPUPPCState *env)
             /* XXX: check this. Seems bugged */
             switch (env->error_code & 0xFF000000) {
             case 0x40000000:
+            case 0x42000000:
                 info.si_signo = TARGET_SIGSEGV;
                 info.si_errno = 0;
                 info.si_code = TARGET_SEGV_MAPERR;