Add team_number intrinsic function

This release is based on the Sourcery Institute GCC fork's teams branch with commits from the GCC git mirror through 16 January 2018 and commits unique to the teams branch through 15 January 2018. This latest release adds support for the Fortran 2018 team_number() intrinsic function. With the support this branch provides for teams, gfortran is approaching full compliance with the Fortran 2018 Draft International Standard, notwithstanding bugs, some partially supported major features, and a few unsupported minor features.