gc/update: replace 'find -print | while read' with 'find -print0 | xargs -0'

gc/update: replace 'find -print | while read' with 'find -print0 | xargs -0'

It's much more effecient and easier to read but it depends on xargs not
doing a broken thing and running the command even when it didn't get
any arguments to run it with.

Fortunately we've previously dealt with that broken xargs behavior
by adding a wrapper to automatically supply the "don't behave wrongly"
option on platforms where it's needed so it's safe to make these changes.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>