Remove references to gdb64 in the testsuite

Remove references to gdb64 in the testsuite

I'm not sure whether this gdb64 was ever a thing in the upstream repo,
but it certainly doesn't exist nowadays.

AFAICT, this came in with the original big merge with the HP tree:
https://sourceware.org/ml/gdb-patches/1999-q2/msg00149.html

gdb/testsuite/ChangeLog:
2017-10-12  Pedro Alves  <palves@redhat.com>

	* gdb.base/attach.exp: Remove references to gdb64.
	* gdb.base/dbx.exp: Remove references to gdb64.