We have a global 1/2 and a cached 1/3, but recalculate 1/4, 1/6 and 1/9
each time we need them.  That seems a bit arbitrary and makes the folding
code more noisy (especially once it's moved to match.pd).

This patch caches the other three constants too.  Bootstrapped &
regression-tested on x86_64-linux-gnu.

gcc/
	* real.h (dconst_quarter, dconst_sixth, dconst_ninth): New macros.
	(dconst_quarter_ptr, dconst_sixth_ptr, dconst_ninth_ptr): Declare.
	* real.c (CACHED_FRACTION): New helper macro.
	(dconst_third_ptr): Use it.
	(dconst_quarter_ptr, dconst_sixth_ptr, dconst_ninth_ptr): New.
	* builtins.c (fold_builtin_sqrt): Use dconst_quarter and
	dconst_sixth.
	(fold_builtin_cbrt): Use dconst_sixth and dconst_ninth.

From-SVN: r228561