gdb/fortran: add support for legacy .xor. operator

gfortran supports .xor. as an alias for .neqv., see:

  https://gcc.gnu.org/onlinedocs/gfortran/_002eXOR_002e-operator.html

this commit adds support for this operator to GDB.

gdb/ChangeLog:

	* f-exp.y (fortran_operators): Add ".xor.".

gdb/testsuite/ChangeLog:

	* gdb.fortran/dot-ops.exp (dot_operations): Test ".xor.".