gdb/fortran: change default logical type to builtin_logical

gdb/fortran: change default logical type to builtin_logical

According to the Fortran standard, logical is of the size of a
'single numeric storage unit' (just like real and integer). For
gfortran, flang and ifx/ifort this storage unit (or the default
logical type) is of size kind 4, actually occupying 4 bytes of
storage, and so the default type for logical expressions in
Fortran should probably also be Logical*4 and not Logical*2.  I
adapted GDB's behavior to be in line with
gfortran/ifort/ifx/flang.