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

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: Thomas Huth <huth@tuxfamily.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>