No code change, only move code from signal.c to
i386/signal.c, except adding includes and
exporting setup_frame() and setup_rt_frame().

x86_64/signal.c includes i386/signal.c
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Message-Id: <20180424192635.6027-17-laurent@vivier.eu>