gdb/fortran: Print 'void' type in lower case

gdb/fortran: Print 'void' type in lower case

For a program compiled with gfortran the base type names are written
as lower cases in the DWARF, and so GDB will display them as lower
case.  Additionally, in most places where GDB supplies its own type
names (for example all of the types defined in f-lang.c in
`build_fortran_types`), the type names are all lower case.

An exception to this is where GDB prints the void type for Fortran.
In this case GDB uses upper case.

I'm not aware of any reason why this type should merit special
attention, and it looks our of place when printing types, so this
commit changes from 'VOID' to 'void' to match all the other types.

gdb/ChangeLog:

	* f-lang.c (build_fortran_types): Change name of void type to
	lower case.
	* f-typeprint.c (f_type_print_base): Print the name of the void
	type, rather than a fixed string.
	* f-valprint.c (f_decorations): Use lower case void string.

gdb/testsuite/ChangeLog:

	* gdb.fortran/exprs.exp (test_convenience_variables): Expect lower
	case void string.