linux-user: set minimum kernel version to 2.6.32

linux-user: set minimum kernel version to 2.6.32

Popular glibc based distributions[1] require minimum
2.6.32 as kernel version. For some targets 2.6.18
would be enough, but dropping so low would mean some
suboptimal system calls could get used.

Set the minimum kernel advertized to 2.6.32 for
all architectures but aarch64 to ensure working qemu
linux-user in case host kernel is older.

[1] https://bugs.launchpad.net/ubuntu/+source/eglibc/+bug/921078

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>