__NR_mmap2 was used for i386 but it's also needed for other archs such
as RISCV32 or ARM. Let's decide to use it based on the __NR_mmap2
definition as it's not defined on other archs.
Signed-off-by: NWilly Tarreau <w@1wt.eu>
Signed-off-by: NPaul E. McKenney <paulmck@kernel.org>