Use file offsets for calculating phdr file size
Do not strip all when stripping an explict symbol

FreeBSD svn r275862:
  When requested to strip specific symbols (-N flag) the default should be
  to strip nothing (other than the requested symbols). This is consistent
  with binutils strip(1).

  PR:           196038
  Reviewed by:  imp
  Sponsored by: The FreeBSD Foundation
  Differential Revision: https://reviews.freebsd.org/D1327