emulators/qemu50: fix build on GCC architectures
Adding -L/usr/lib make gcc from ports prefer /usr/lib instead of its own /usr/local/lib/gcc9, which makes build fail:
/usr/local/bin/ld: accel/tcg/tcg-runtime.o: in function `clrsb32':
/wrkdirs/usr/ports/emulators/qemu50/work/qemu-5.0.0/include/qemu/host-utils.h:211: undefined reference to `__clrsbdi2'
Tested for no breakage on 12.1/i386, 12.2/amd64 and 13.0/powerpc64.
Approved by: portmgr (fix build blanket)
8f2be5ce19