Allow copy-builtin to work with modified sources

Allow copy-builtin to work with modified sources

`scripts/make_gitrev.sh` had 'set -e' so if any command failed it would
fail and cause copy-builtin to fail (copy-builtin also has `set -e`.
This commit also simplifies scripts/make_gitrev.sh to always write a
file by using a cleanup function.  It also simplifies other areas of
the script as well (making it much shorter).

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Thode <mthode@mthode.org>
Closes #8022
Closes #8025