Add a build_real_truncate helper function

Add a build_real_truncate helper function

...which simplifies the match.pd patterns I'm about to add.

Bootstrapped & regression-tested on x86_64-linux-gnu.

gcc/
	* real.h (build_real_truncate): Declare.
	* tree.c (build_real_truncate): New function.
	(strip_float_extensions): Use it.
	* builtins.c (fold_builtin_cabs, fold_builtin_sqrt, fold_builtin_cbrt)
	(fold_builtin_hypot, fold_builtin_pow): Likewise.
	* match.pd: Likewise.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@228483 138bc75d-0d04-0410-961f-82ee72b054a4