Divide hw.memsize by 1073741824 for expected GB results

Dividing the number of bytes returned from `sysctl hw.memsize` by
1.0x10^9 gives unexpected results on machines with a large amount of
RAM. For example, on a 16GB system, `archey` will report **17 GB**.
Dividing by 1073741824 (i.e., 1024^3) yields the expected results.