strconv: revert ParseFloat/ParseComplex error on incorrect bitSize

This is a partial revert of https://go-review.googlesource.com/c/go/+/248219
because we found that a non-trivial amount of code erroneously calls
ParseFloat(s, 10) or even ParseFloat(s, 0) and expects it to work --
before that change was merged, ParseFloat accepted a bitSize of
anything other than 32 or 64 to mean 64 (and ParseComplex was similar).

So revert that behavior to avoid breaking people's code, and add tests
for this.

I may add a vet check to flag ParseFloat(s, not_32_or_64) in a later
change.

See #42297 for more details.

Change-Id: I4bc0156bd74f67a39d5561b6e5fde3f2d20bd622
Reviewed-on: https://go-review.googlesource.com/c/go/+/267319
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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