patch from paul eggert to fix -0.0/0.0 confusion that breaks glibc

patch from paul eggert to fix -0.0/0.0 confusion that breaks glibc
	* real.h (REAL_VALUES_IDENTICAL): New macro.
	* expr.c (is_zeros_p): Don't consider -0.0 to be all zeros.
	* fold-const.c (operand_equal_p): Don't consider -0.0 to be
	identical to 0.0.
	* tree.c (simple_cst_equal): Don't consider -0.0 to have the
	same tree structure as 0.0.
	* varasm.c (immed_real_const_1): Use new REAL_VALUES_IDENTICAL
	macro instead of doing it by hand.

From-SVN: r16472