Mk/Uses/fortran.mk: Add flang as a non-default USES=fortran argument. Note,
flang is currently only available for amd64.

PR: 220313
Submitted by: yuri@rawbw.com
Reviewed by: db, jmd, mat, swills
Approved by: portmgr
Differential Revision: https://reviews.freebsd.org/D12529