math/flexiblas: fix build on platforms without OpenMP (armv7)
On such platforms, openblas uses pthreads over OpenMP. Add some more
switches to pkg-plist to address this.
Fix a typo in pkg-descr while we are at it.
Approved by: portmgr (build fix blanket)
MFH: 2026Q2
(cherry picked from commit b9b8097cb0139bd2bbbc22b810d40d8a266841f9)