linux-user: avoid using glibc internals in _syscall5 and in definition of target_sige...

linux-user: avoid using glibc internals in _syscall5 and in definition of target_sigevent struct

Use the public sigset_t instead of the glibc specific internal
__sigset_t in _syscall.

Calculate the sigevent pad size is calculated in similar way as kernel
does it instead of using glibc internal field _pad.

This is needed for building with musl libc.

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>