Fix gdb.base/whatis-ptype-typedefs.exp on 32-bit archs

Fix gdb.base/whatis-ptype-typedefs.exp on 32-bit archs

The gdb.base/whatis-ptype-typedefs.exp testcase has several tests that
fail on 32-bit architectures.  E.g., on 'x86-64 -m32', I see:

 ...
 FAIL: gdb.base/whatis-ptype-typedefs.exp: lang=c: cast: whatis (float_typedef) v_uchar_array_t_struct_typedef (invalid)
 FAIL: gdb.base/whatis-ptype-typedefs.exp: lang=c: cast: ptype (float_typedef) v_uchar_array_t_struct_typedef (invalid)
 ...

gdb.log:

 (gdb) whatis (float_typedef) v_uchar_array_t_struct_typedef
 type = float_typedef
 (gdb) FAIL: gdb.base/whatis-ptype-typedefs.exp: lang=c: cast: whatis (float_typedef) v_uchar_array_t_struct_typedef (invalid)

As Simon explained [1], the issue boils down to the fact that on
64-bit, this is an invalid cast:

 (gdb) p (float_typedef) v_uchar_array_t_struct_typedef
 Invalid cast.

while on 32 bits it is valid:

 (gdb) p (float_typedef) v_uchar_array_t_struct_typedef
 $1 = 1.16251721e-41

The expression basically tries to cast an array (which decays to a
pointer) to a float.  The cast works on 32 bits because a float and a
pointer are of the same size, and value_cast works in that case:

~~~
   More general than a C cast: accepts any two types of the same length,
   and if ARG2 is an lvalue it can be cast into anything at all.  */
~~~

On 64 bits, they are not the same size, so it ends throwing the
"Invalid cast" error.

The testcase is expecting the invalid cast behavior, thus the FAILs.

A point of these tests was to cover as many code paths in value_cast
as possible, as a sort of documentation of the current behavior:

    # The main idea here is testing all the different paths in the
    # value casting code in GDB (value_cast), making sure typedefs are
    # preserved.
...
    # We try all combinations, even those that don't parse, or are
    # invalid, to catch the case of a regression making them
    # inadvertently valid.  For example, these convertions are
    # invalid:
...

In that spirit, this commit makes the testcase adjust itself depending
on size of floats and pointers, and also test floats of different
sizes.

Passes cleanly on x86-64 GNU/Linux both -m64/-m32.

[1] - https://sourceware.org/ml/gdb-patches/2017-11/msg00382.html

gdb/ChangeLog:
2017-11-20  Pedro Alves  <palves@redhat.com>

	* gdb.base/whatis-ptype-typedefs.c (double_typedef)
	(long_double_typedef): New typedefs.
	Use DEF on double and long double.
	* gdb.base/whatis-ptype-typedefs.exp: Add double and long double
	cases.
	(run_tests): New 'float_ptr_same_size', 'double_ptr_same_size',
	and 'long_double_ptr_same_size' locals.  Use them to decide
	whether cast from array/function to float is valid/invalid.