tools/nolibc/sys: split the syscall definitions into their own file

The syscall definitions were moved to sys.h. They were arranged
in a more easily maintainable order, whereby the sys_xxx() and xxx()
functions were grouped together, which also enlights the occasional
mappings such as wait relying on wait4().

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>