re PR middle-end/29335 (transcendental functions with constant arguments should be resolved at compile-time)

	PR middle-end/29335
	* builtins.c (fold_builtin_1): Evaluate tgamma using MPFR.

testsuite:
	* gcc.dg/torture/builtin-math-2.c: Add tgamma tests.
	* gcc.dg/torture/builtin-math-3.c: Likewise.

From-SVN: r118200