Fix FMA_NATIVE constant

[simonbyrne]

As pointed out be @StefanKarpinski in #31922 (comment)

[chriselrod]

Is it really fixed?

julia> Base.Math.FMA_NATIVE
false

julia> @code_native debuginfo=:none fma(1.0, 2.0, 3.0)
        .text
        vfmadd213sd     %xmm2, %xmm1, %xmm0 # xmm0 = (xmm1 * xmm0) + xmm2
        retq
        nopw    %cs:(%rax,%rax)

julia> versioninfo()
Julia Version 1.5.0-DEV.362
Commit 6dd9f418ef (2020-02-28 22:34 UTC)
Platform Info:
  OS: Linux (x86_64-generic-linux)
  CPU: Intel(R) Core(TM) i9-10980XE CPU @ 3.00GHz

EDIT:
Although, I am not sure what is going on, because:

julia> muladd(nextfloat(1.0),nextfloat(1.0),-nextfloat(1.0,2))
4.930380657631324e-32

julia> muladd(nextfloat(1.0),nextfloat(1.0),-nextfloat(1.0,2)) != 0
true

Anyone have any idea how I can investigate this?

[KristofferC]

#33011