gdb/testsuite/lib/dwarf.exp: fix addr_size parameter comments

gdb/testsuite/lib/dwarf.exp: fix addr_size parameter comments

The comments modified in this patch claim that the addr_size parameters
can take the value 32 or 64 (suggesting the value is in bits).  In fact,
the expected value is in bytes, either 4 or 8.

The actual value in the DWARF info is in bytes.  And we can see that the
default values used (if addr_size == "default") are:

	if {$_cu_addr_size == "default"} {
	    if {[is_64_target]} {
		set _cu_addr_size 8
	    } else {
		set _cu_addr_size 4
	    }
	}

gdb/testsuite/ChangeLog:

	* lib/dwarf.exp (Dwarf::cu, Dwarf::tu, Dwarf::lines): Change valid
	values in documentation for addr_size to 4 and 8.

Change-Id: I4a02dca2bb7992198864e545ef099f020f54ff2f