fmaf is sometimes incorrect

This example was pointed out by njuffa on scicomp.stackexchange.com. I checked it also with a recent version of Z3 and exact rational arithmetic.

```
# This is the native cpu instruction
julia> fma(0.9474001f0, 4.639901f-7, -0.24325085f0)
-0.24325041f0

julia> Base.fma_libm(0.9474001f0, 4.639901f-7, -0.24325085f0)
-0.2432504f0

julia> versioninfo()
Julia Version 0.6.0
Commit 903644385b (2017-06-19 13:05 UTC)
Platform Info:
OS: macOS (x86_64-apple-darwin13.4.0)
CPU: Intel(R) Core(TM) i7-4770HQ CPU @ 2.20GHz
WORD_SIZE: 64
BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Haswell)
LAPACK: libopenblas64_
LIBM: libopenlibm
LLVM: libLLVM-3.9.1 (ORCJIT, haswell)
```