libstdc++: add attributes to more ios_base types

libstdc++: add attributes to more ios_base types

In r15-3499 I added attributes to _Ios_Openmode to avoid -Wswitch false
positives; let's do the same for the other enums in ios_base.

It also seems to me that with these attributes, the tests don't need to
include the end/max/min cases.

libstdc++-v3/ChangeLog:

	* include/bits/ios_base.h: Add attribs to _Ios_Fmtflags,
	_Ios_Iostate, _ios_seekdir.
	* testsuite/27_io/ios_base/types/fmtflags/case_label.cc: Remove
	unneeded cases.
	* testsuite/27_io/ios_base/types/iostate/case_label.cc: Likewise.
	* testsuite/27_io/ios_base/types/openmode/case_label.cc: Likewise.
	* testsuite/27_io/ios_base/types/seekdir/case_label.cc: Likewise.