Support long double values

[eyalroz]

We currently don't support long double floating-point values - neither in terms of the printf syntax (%Lf is rejected), nor in terms of the semantics (we never va_arg(args, long double)).

As some platforms, and compilers, actually support long doubles - we should do so as well. At first, at least with lower-precision printing by casting to a double, but then preferably printing proper long doubles.

[eyalroz]

So, the syntax is now supported. The harder part is actually processing long doubles.