Implement the missing __floattitf() and __floatuntitf() functions, to

convert 128-bit (unsigned) integers to quad-precision floating-point

types. This is needed e.g. on AArch64 where 'long double' is

a quad-precision type.

The code is based on the existing code for __floattixf()
and __floatuntixf(), updated to account for different bit field lengths

of quad-precision float. The tests are also copied, with the rounding

tests adjusted for longer significand.

(tested on AArch64)