strconv: simplify (*extFloat).Multiply using math/bits.Mul64

strconv: simplify (*extFloat).Multiply using math/bits.Mul64

This method was using a handwritten long multiplication of uint64s.
Since implementation of #24813 we can remove it and replace it
by Mul64 from math/bits.

This brings a small speedup for 64-bit platforms.

Benchmarks on Haswell Celeron 2955U.

benchmark                                  old ns/op     new ns/op     delta
BenchmarkAppendFloat/Decimal-2             127           127           +0.00%
BenchmarkAppendFloat/Float-2               340           317           -6.76%
BenchmarkAppendFloat/Exp-2                 258           233           -9.69%
BenchmarkAppendFloat/NegExp-2              256           231           -9.77%
BenchmarkAppendFloat/Big-2                 402           375           -6.72%
BenchmarkAppendFloat/BinaryExp-2           113           114           +0.88%
BenchmarkAppendFloat/32Integer-2           125           125           +0.00%
BenchmarkAppendFloat/32ExactFraction-2     274           249           -9.12%
BenchmarkAppendFloat/32Point-2             339           317           -6.49%
BenchmarkAppendFloat/32Exp-2               255           229           -10.20%
BenchmarkAppendFloat/32NegExp-2            254           229           -9.84%
BenchmarkAppendFloat/64Fixed1-2            165           154           -6.67%
BenchmarkAppendFloat/64Fixed2-2            184           176           -4.35%
BenchmarkAppendFloat/64Fixed3-2            168           158           -5.95%
BenchmarkAppendFloat/64Fixed4-2            187           177           -5.35%
BenchmarkAppendFloat/Slowpath64-2          84977         84883         -0.11%

Change-Id: If05784e856289b3b7bf136567882e7ee10234756
Reviewed-on: https://go-review.googlesource.com/c/go/+/157717
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>