linux-user: fix do_rt_sigreturn on m68k linux userspace emulation

do_rt_sigreturn uses an uninitialised local variable instead of fetching
the old signal mask directly from the signal frame when restoring the mask,
so the signal mask is undefined after do_rt_sigreturn. As the signal
frame data is in target-endian order, target_to_host_sigset instead of
target_to_host_sigset_internal is required.

do_sigreturn is correct in using target_to_host_sigset_internal, because
get_user already did the endianness conversion.

Signed-off-by: Michael Karcher <karcher@physik.fu-berlin.de>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20170225110517.2832-3-laurent@vivier.eu>
