strconv: fix Eisel-Lemire for negative zero
strconv: fix Eisel-Lemire for negative zero

This is somewhat academic (and no tests failed before this commit),
since func atof64 only calls func eiselLemire when func atof64exact
fails, and func atof64exact doesn't fail when parsing positive or
negative zeroes. But it's still worth fixing.

Change-Id: Ibe6ef4c8fd96827673b711d5456003fbc447e39c
Reviewed-on: https://go-review.googlesource.com/c/go/+/264140
Trust: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

cloudflare/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,golang/go,CAFxX/go