graphics/gimp3-app: fix build on armv7
No OpenMP on armv7 unfortunately.
While we are at it, rework the "no FP exceptions" patch to use the
newer <fenv.h> interface, as the old one is not supported everywhere.
Approved by: portmgr (build fix blanket)
MFH: 2025Q4
(cherry picked from commit 927fbf0126e89f055975430574ac1d2d968fe25b)