Fix gdb.ada/packed_array_assign.exp by using more unique names.

Fix gdb.ada/packed_array_assign.exp by using more unique names.

The test gdb.ada/packed_array_assign fails due to conflict between component 'w'
and system.dim.mks.w:

(gdb) print pra := ((x => 2, y => 0, w => 17), pr, (x => 7, y => 1, w => 23))
Unknown component name: system.dim.mks.w.
(gdb) FAIL: gdb.ada/packed_array_assign.exp: print pra := ((x => 2, y => 0, w => 17), pr, (x => 7, y => 1, w => 23))

Also, depending on the compiler version, the component w might be reordered
and placed before components x and y.
So, change the component order in the source, so that both an old
compiler (GNATMAKE 6.3.0, gcc (Debian 6.3.0-18+deb9u1) 6.3.0 20170516)
and a new compiler (GNATMAKE Pro 20.0w (20181210-82), based on gcc 8.2.1)
produce the same component order (checked by using -gnatR3s).

So, update to test the new (more unique) names in the source order.

2018-12-26  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* gdb.ada/packed_array_assign/aggregates.ads (Packed_Rec):
	Rename components to Packed_Array_Assign_[X|Y|W].  Place
	component Packed_Array_Assign_W as first component, to ensure
	old and new compilers have the same representation.
	All users updated.