[libc][math] Improve fmul performance by using double-double arithmetic. (#107517)
```
Performance tests with inputs in denormal range:
-- My function --
Total time : 2731072304 ns
Average runtime : 68.2767 ns/op
Ops per second : 14646276 op/s
-- Other function --
Total time : 3259744268 ns
Average runtime : 81.4935 ns/op
Ops per second : 12270913 op/s
-- Average runtime ratio --
Mine / Other's : 0.837818
Performance tests with inputs in normal range:
-- My function --
Total time : 93467258 ns
Average runtime : 2.33668 ns/op
Ops per second : 427957777 op/s
-- Other function --
Total time : 637295452 ns
Average runtime : 15.9324 ns/op
Ops per second : 62765299 op/s
-- Average runtime ratio --
Mine / Other's : 0.146662
Performance tests with inputs in normal range with exponents close to each other:
-- My function --
Total time : 95764894 ns
Average runtime : 2.39412 ns/op
Ops per second : 417690014 op/s
-- Other function --
Total time : 639866770 ns
Average runtime : 15.9967 ns/op
Ops per second : 62513075 op/s
-- Average runtime ratio --
Mine / Other's : 0.149664
```
---------
Co-authored-by: Tue Ly <lntue@google.com>
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