Add support for USE_GCC=5 and its preferred form USE_GCC=5+.

PR:		194676


git-svn-id: svn+ssh://svn.freebsd.org/ports/head@372091 35697150-7ecd-e111-bb59-0022644237b5
- Finish conversion of NVVERSION to float (left-pad minor < 99 with zero
  to allow version comparison); now we're safe until minor wraps to 1000
- Augment a comment about NVVERSION and reword PAE option description
- Make `update-distinfo' target more robust (by respecting ARCH_SUFX)


git-svn-id: svn+ssh://svn.freebsd.org/ports/head@372092 35697150-7ecd-e111-bb59-0022644237b5