libstdc++: Use _GLIBCXX_USE_BUILTIN_TRAIT

This patch uses _GLIBCXX_USE_BUILTIN_TRAIT macro instead of __has_builtin
in the type_traits header for traits that have a corresponding fallback
non-built-in implementation.  This macro supports to toggle the use of
built-in traits in the type_traits header through
_GLIBCXX_DO_NOT_USE_BUILTIN_TRAITS macro, without needing to modify the
source code.

libstdc++-v3/ChangeLog:

	* include/std/type_traits: Use _GLIBCXX_USE_BUILTIN_TRAIT.

Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>