Use SYS_SETUID32 for system.Setuid() on Linux for ARM

Besides possible other problems 16bit UIDs might be disabled by the kernel
configuration on Linux for ARM (see CONFIG_UID16).

Signed-off-by: Alexander Holler <holler@ahsoftware.de>
Merge pull request #269 from aholler/master

Use SYS_SETUID32 for system.Setuid() on Linux for ARM