Add support for __builtin_bswap128

Add support for __builtin_bswap128

This patch introduces a new builtin named __builtin_bswap128 on targets
where TImode is supported, i.e. 64-bit targets only in practice.  The
implementation simply reuses the existing double word path in optab, so
no routine is added to libgcc (which means that you get two calls to
_bswapdi2 in the worst case).

gcc/ChangeLog:

	* builtin-types.def (BT_UINT128): New primitive type.
	(BT_FN_UINT128_UINT128): New function type.
	* builtins.def (BUILT_IN_BSWAP128): New GCC builtin.
	* doc/extend.texi (__builtin_bswap128): Document it.
	* builtins.c (expand_builtin): Deal with BUILT_IN_BSWAP128.
	(is_inexpensive_builtin): Likewise.
	* fold-const-call.c (fold_const_call_ss): Likewise.
	* fold-const.c (tree_call_nonnegative_warnv_p): Likewise.
	* tree-ssa-ccp.c (evaluate_stmt): Likewise.
	* tree-vect-stmts.c (vect_get_data_ptr_increment): Likewise.
	(vectorizable_call): Likewise.
	* optabs.c (expand_unop): Always use the double word path for it.
	* tree-core.h (enum tree_index): Add TI_UINT128_TYPE.
	* tree.h (uint128_type_node): New global type.
	* tree.c (build_common_tree_nodes): Build it if TImode is supported.

gcc/testsuite/ChangeLog:

	* gcc.dg/builtin-bswap-10.c: New test.
	* gcc.dg/builtin-bswap-11.c: Likewise.
	* gcc.dg/builtin-bswap-12.c: Likewise.
	* gcc.target/i386/builtin-bswap-5.c: Likewise.