tools/nolibc/sys: make open() take a vararg on the 3rd argument

tools/nolibc/sys: make open() take a vararg on the 3rd argument

Let's pass a vararg to open() so that it remains compatible with existing
code. The arg is only dereferenced when flags contain O_CREAT. The function
is generally not inlined anymore, causing an extra call (total 16 extra
bytes) but it's still optimized for constant propagation, limiting the
excess to no more than 16 bytes in practice when open() is called without
O_CREAT, and ~40 with O_CREAT, which remains reasonable.

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