Since clang supports it and there is no equivalent 128-bit type currently supported by clang, please enable it.
In the current ML conversation it has been revealed: "But from clang's OSTargets.h file, it looks like 128 bit float support is never set to enabled for FreeBSD." 
 hackers@ thread: https://lists.freebsd.org/pipermail/freebsd-hackers/2019-May/054660.html
Is there some reason the compiler should emulate a floating point mode that doesn't exist on any hardware, rather than using a separate library like MPFR?
(In reply to Conrad Meyer from comment #1)
IMO, this question should be directed to the clang team. Once they have some feature we shouldn't be second guessing their judgement.
As mentioned on the mailing list, it is not a matter of just "enabling" float128. Somebody has to step up and write a BSD licensed quadmath.h, to start with.