Do not emit "field_type" var if not needed on "maint print c-tdesc"

While fiddling a bit with -Wunused-variable, Sergio noticed that "maint
print c-tdesc" was always generating code for the "tdesc_type
*field_type" variable, even when it wasn't used.  This is caught by GCC
when using -Wunused-variable, of course.

This patch changes the print_c_tdesc class to only output the field
declaration when we actually need it.

It shouldn't be necessary to do the same with the other variable
declarations (type_with_fields and element_type), because they are
always if they are declared.

The C files in features/ are regenerated, some declarations of
field_type are removed, as expected, while some others move to where
they are used for the first time.

gdb/ChangeLog:

	* target-descriptions.c (print_c_tdesc) <visit>: Don't output
	field_type declaration, use printf_field_type_assignment
	instead.
	<printf_field_type_assignment>: New method.
	* features/aarch64-core.c, features/aarch64-fpu.c
	features/arc-arcompact.c, features/arc-v2.c,
	features/arm/arm-with-iwmmxt.c, features/i386/32bit-core.c,
	features/i386/32bit-mpx.c, features/i386/32bit-sse.c,
	features/i386/64bit-avx512.c, features/i386/64bit-core.c,
	features/i386/64bit-mpx.c, features/i386/64bit-sse.c,
	features/i386/x32-core.c, features/or1k.c,
	features/rs6000/powerpc-7400.c,
	features/rs6000/powerpc-altivec32.c,
	features/rs6000/powerpc-altivec32l.c,
	features/rs6000/powerpc-altivec64.c,
	features/rs6000/powerpc-altivec64l.c,
	features/rs6000/powerpc-cell32l.c,
	features/rs6000/powerpc-cell64l.c,
	features/rs6000/powerpc-isa205-altivec32l.c,
	features/rs6000/powerpc-isa205-altivec64l.c,
	features/rs6000/powerpc-isa205-vsx32l.c,
	features/rs6000/powerpc-isa205-vsx64l.c,
	features/rs6000/powerpc-vsx32.c,
	features/rs6000/powerpc-vsx32l.c,
	features/rs6000/powerpc-vsx64.c,
	features/rs6000/powerpc-vsx64l.c, features/s390-gs-linux64.c,
	features/s390-tevx-linux64.c, features/s390-vx-linux64.c,
	features/s390x-gs-linux64.c, features/s390x-tevx-linux64.c,
	features/s390x-vx-linux64.c: Re-generate.

gdb/features/rs6000/powerpc-vsx32l.c

@@ -108,8 +108,8 @@ initialize_tdesc_powerpc_vsx32l (void)
   tdesc_create_vector (feature, "v16i8", element_type, 16);
 
   tdesc_type_with_fields *type_with_fields;
-  tdesc_type *field_type;
   type_with_fields = tdesc_create_union (feature, "vec128");
+  tdesc_type *field_type;
   field_type = tdesc_named_type (feature, "uint128");
   tdesc_add_field (type_with_fields, "uint128", field_type);
   field_type = tdesc_named_type (feature, "v4f");