System arch check on Debian/Ubuntu

[Cafuego]

The installer incorrectly uses `uname -m' to check the arch on Debian and Ubuntu systems. The problem is that this returns the kernel arch, which is not necessarily the same as the userspace arch.

For example, I run a 64bit Xen system with a 32bit domU for Zimbra, as back when the system was first installed there was no 64bit ZCS.

By having get_plat_tags.sh query the package system for the userspace arch instead of the kernel arch, the --platform-override flag is not required, as the installer matches the arch correctly.

Patch attached.