[testsuite] dw2-const.S: fix type reference size on 64bit platforms.

[testsuite] dw2-const.S: fix type reference size on 64bit platforms.

The testcase was failing on various 64bit platforms, because the
debugging info said that the target type for a const type is a
4-byte reference.  In the asm file, I used a .long directive, but
the size of .long is of course dependent on the platform.  This
replaces this .long by a .4byte directive.

gdb/testsuite/ChangeLog:

        * gdb.dwarf2/dw2-const.S: Use .4byte to reference the target
        type of our const type.