Fix gdb.python/py-format-string.exp with Clang

Fix gdb.python/py-format-string.exp with Clang

GDB includes the virtual table pointer when formatting polymorphic
C++ objects for printing, but GCC and Clang name these differently:
GCC emits a DW_AT_name of "_vptr.Base" when describing the virtual
table pointer of a type derived from type "Base", whereas Clang
will emit "_vptr$Base" in this situation.  This commit fixes a
testcase which failed because of this.

gdb/testsuite/ChangeLog:

	* gdb.python/py-format-string.exp (test_deref_refs): Treat
	"_vptr$Base" as correct, in addition to "_vptr.Base".
	(test_mixed): Likewise.