Move pow folds to match.pd

Move pow folds to match.pd

Tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabi.

gcc/
	* builtins.c (fold_builtin_pow): Delete in favor of...
	(fold_const_builtin_pow): ...this new function.  Only handle constant
	arguments.
	(fold_builtin_2): Update accordingly.
	* match.pd: Add rules previously handled by fold_builtin_pow.

gcc/testsuite/
	* gcc.dg/torture/builtin-math-1.c: Skip at -O0.

From-SVN: r229408