signal/all: remove __get/__put_user return value reading

Remove all the simple cases of reading the return value
of __get_user and __put_user.

We set err = 0 in sparc versions of do_sigreturn and
sparc64_set_context to avoid compile error, but else this patch is
just general removal of err |= __get_user ... idiom.

v2: remove err variable from target_rt_restore_ucontext
Signed-off-by: NRiku Voipio <riku.voipio@linaro.org>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>