Improve STRIP_DEBUG

[Jonathan Perkin <jperkin%joyent.com@localhost>]

I've been working on adding CTF support to pkgsrc, and during that
work I hit a few issues with our current STRIP_DEBUG support.

This diff fixes them, the details are in the commit log:

  https://github.com/joyent/pkgsrc/commit/f869b311e4d49fcc976018ed06b2996a06f95b78

I've used this successfully for our 2017Q4 builds where packages are
built with debug, then converted to CTF, and finally stripped to
reduce size while retaining CTF information.

Please review, thanks.

-- 
Jonathan Perkin  -  Joyent, Inc.  -  www.joyent.com