x11-toolkits/qt5-gui: unbreak on FreeBSD >= 13.0 armv[67]

/usr/bin/cc ... -fno-integrated-as ... ../3rdparty/pixman/pixman-arm-neon-asm.S -o .obj/pixman-arm-neon-asm.o
cc: error: unable to execute command: Executable "as" doesn't exist!
cc: error: assembler command failed with exit code 1 (use -v to see invocation)

Reported by: pkg-fallout

(cherry picked from commit 7ebdbb52f8d7050fcfd6bbad5eac0ade8a52597e)