sys-apps/portage: version bump to 2.3.21

sys-apps/portage: version bump to 2.3.21

  #612972 - fix global scope DISTDIR setting to be consistent
  #645416 - dep_zapdeps: fix virtual/rust handling
  #645780 - add --changed-deps-report option

Package-Manager: Portage-2.3.20, Repoman-2.3.6

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# Date:      Mon Jan 29 22:18:27 2018 -0800
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# On branch master
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#	modified:   Manifest
#	new file:   portage-2.3.21.ebuild
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