MFC r270417, r270418, r270455, r270457:

r270417:
  Fix arm build breakage when building stable/10 on
  head/.

r270418:
  Also export UNAME_r to fix arm builds.

r270455:
  Set OSREL and UNAME_r in release/release.sh when building
  ports to prevent ports build failures from killing the
  release build.

r270457:
  Wrap a long line.

Sponsored by:	The FreeBSD Foundation