mk/compiler/gfortran.mk: bump default from gcc9 to gcc10 on current aarch64

lang/gcc8 has patches for NetBSD/aarch64 and lang/gcc10 has support mostly
upstreamed. Nobody seems interested in fixing gcc9, but the pkgsrc
logic defaults to it when the system compiler is GCC 9 which leads to
broken fortran packages. Let's just skip forward to gcc10.

(tnn)