libstdc++: Fix some tests that fail with -fno-exceptions

libstdc++: Fix some tests that fail with -fno-exceptions

libstdc++-v3/ChangeLog:

	* testsuite/18_support/nested_exception/rethrow_if_nested-term.cc:
	Require effective target exceptions_enabled instead of using
	dg-skip-if.
	* testsuite/23_containers/vector/capacity/constexpr.cc: Expect
	shrink_to_fit() to be a no-op without exceptions enabled.
	* testsuite/23_containers/vector/capacity/shrink_to_fit.cc:
	Likewise.
	* testsuite/ext/bitmap_allocator/check_allocate_max_size.cc:
	Require effective target exceptions_enabled.
	* testsuite/ext/malloc_allocator/check_allocate_max_size.cc:
	Likewise.
	* testsuite/ext/mt_allocator/check_allocate_max_size.cc:
	Likewise.
	* testsuite/ext/new_allocator/check_allocate_max_size.cc:
	Likewise.
	* testsuite/ext/pool_allocator/check_allocate_max_size.cc:
	Likewise.
	* testsuite/ext/throw_allocator/check_allocate_max_size.cc:
	Likewise.