fix dwz.exp on 32-bit targets

fix dwz.exp on 32-bit targets

This fixes dwz.exp on 32-bit targets.  It does so by introducing a new
"default" setting for the address size in the DWARF assembler.

Built and regtested on x86-64 Fedora 18.
I also ran the gdb.dwarf2 tests on an x86 machine (gcc45).

	* lib/dwarf.exp (cu, tu): Handle addr_size of "default".  Change
	default addr_size.
	* lib/gdb.exp (is_64_target): New gdb_caching_proc.