[gdb/testsuite] Skip gdb.fortran/namelist.exp for gfortran 4.8

[gdb/testsuite] Skip gdb.fortran/namelist.exp for gfortran 4.8

The test-case gdb.fortran/namelist.exp uses a gfortran feature (emitting
DW_TAG_namelist in the debug info) that has been supported since gfortran 4.9,
see PR gcc/37132.

Skip the test for gfortran 4.8 and earlier.  Do this using gcc_major_version,
and update it to be able to handle "gcc_major_version {gfortran-*} f90".

Tested on x86_64-linux, with gfortran 4.8.5, 7.5.0, and 12.1.1.