re PR c++/8861 ([ABI] mangling floating point literal in template arg expression)

	PR 8861
	* mangle.c (write_real_cst): New function.  Implement
	ABI-compliant mangling of floating-point literals when
	-fabi-version>=2; provide backward compatibility with 3.3 when
	-fabi-version=1 (with warning).  Clarify commentary.
	(write_template_arg_literal): Use write_real_cst.

From-SVN: r67670