(#18393) AIX 5.3 doesn't support diff -u

Prior to this commit, the diffargs where always `-u`, which is not
supported on AIX 5.3, an official PE supported platform. This commit
adds a utility method to abstract away the logic for what the default
diffargs should be and then calls that for the default value.
Merge pull request #1637 from jeffweiss/tickets/bug/18393_diffargs_broken_on_aix53

(#18393) AIX 5.3 doesn't support diff -u