turn off LLVM FMA discovery
turn off LLVM FMA discovery

This helps guarantee same-machine consistency
between the interpreter and the JIT.

2-3% perf loss on exemplar benches, which we think we can
recover with the following TODO:

TODO(herb): add Op::{fnma,fms,fnms}_f32 for the various ±xy ± z
combinations we can generate in the backends, peephole, etc.
just as we do Op::fma_f32 so we can recover the (tiny) performance
loss here we take from losing those discovered fused instructions.

Change-Id: Id30e1b9178a55e8072a30d2a34e7ed9188543d60
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/275139
Commit-Queue: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>
Reviewed-by: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>

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