linux-user,target-ppc: fix use of MSR_LE

setup_frame()/setup_rt_frame()/restore_user_regs() are using
MSR_LE as the similar kernel functions do: as a bitmask.

But in QEMU, MSR_LE is a bit position, so change this
accordingly.

The previous code was doing nothing as MSR_LE is 0,
and "env->msr &= ~MSR_LE" doesn't change the value of msr.

And yes, a user process can change its endianness,
see linux kernel commit:

    fab5db9 [PATCH] powerpc: Implement support for setting little-endian mode via prctl

and prctl(2): PR_SET_ENDIAN, PR_GET_ENDIAN
Reviewed-by: NThomas Huth <huth@tuxfamily.org>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>
Signed-off-by: NRiku Voipio <riku.voipio@linaro.org>

linux-user/signal.c

@@ -4632,7 +4632,7 @@ static void restore_user_regs(CPUPPCState *env,
 
     /* If doing signal return, restore the previous little-endian mode.  */
     if (sig)
-        env->msr = (env->msr & ~MSR_LE) | (msr & MSR_LE);
+        env->msr = (env->msr & ~(1ull << MSR_LE)) | (msr & (1ull << MSR_LE));
 
     /* Restore Altivec registers if necessary.  */
     if (env->insns_flags & PPC_ALTIVEC) {
@@ -4747,7 +4747,7 @@ static void setup_frame(int sig, struct target_sigaction *ka,
 #endif
 
     /* Signal handlers are entered in big-endian mode.  */
-    env->msr &= ~MSR_LE;
+    env->msr &= ~(1ull << MSR_LE);
 
     unlock_user_struct(frame, frame_addr, 1);
     return;
@@ -4842,7 +4842,7 @@ static void setup_rt_frame(int sig, struct target_sigaction *ka,
 #endif
 
     /* Signal handlers are entered in big-endian mode.  */
-    env->msr &= ~MSR_LE;
+    env->msr &= ~(1ull << MSR_LE);
 
     unlock_user_struct(rt_sf, rt_sf_addr, 1);
     return;