Builtins that don't work

[David Edelsohn]

>>>>> Mark Mitchell writes:

Mark> It doesn't solve the underlying issue (should __builtin_sqrtf
Mark> even exist on a target without sqrtf?), but it definitely looks like it
Mark> should solve the PR that inspired 3474.  Thanks for the pointer; I will
Mark> verify that it is fixed.

	This is the same question that I asked Gaby.  I had suggested
modifying std_cmath.h to perform the static conversion if the system did
not provide sqrtf, but I understood Gaby's reply to me earlier to mean
that those __builtin_xxx functions must be available unconditionally in
std_cmath.h.  I may have misunderstood.

David