libstdc++ testsuite: Don't run lwg3464.cc tests on simulators

libstdc++ testsuite: Don't run lwg3464.cc tests on simulators

These tests have always been failing for my autotester running a
cris-elf simulator; when unrestrained they take about 20 minutes each,
compared to the (doubled) timeout of 720 seconds, of a total 2h40min
for the whole of the libstdc++-v3 testsuite.  The tests cover counter
overflow and are already disabled for LP64 targets.

	* testsuite/27_io/basic_istream/get/char/lwg3464.cc: Don't run on
	simulator targets.
	* testsuite/27_io/basic_istream/get/wchar_t/lwg3464.cc: Likewise.