Many architectures duplicate similar shell scripts.

This commit converts ARM to use scripts/syscallhdr.sh, and also
collects OABI/OEBI syscalls into unistd-eabi.h/unistd-oabi.h,
removing unistd-common.h.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>