linux-user: Use safe_syscall for futex syscall
Use the safe_syscall wrapper for the futex syscall.
In particular, this fixes hangs when using programs that link
against the Boehm garbage collector, including the Mono runtime.
(We don't change the sys_futex() call in the implementation of
the exit syscall, because as the FIXME comment there notes
that should be handled by disabling signals, since we can't
easily back out if the futex were to return ERESTARTSYS.)
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>