net/mail: lazily initialize dateLayouts
net/mail: lazily initialize dateLayouts

Saves 6KB of memory in stdlib packages.

Updates #26775

Change-Id: I1a6184cefa78e9a3c034fa84506fdfe0fec27add
Reviewed-on: https://go-review.googlesource.com/127736
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

josharian/go,akutz/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,josharian/go,cloudflare/go,CAFxX/go,akutz/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,akutz/go,golang/go,josharian/go,akutz/go,golang/go,cloudflare/go,akutz/go,akutz/go,josharian/go,nawawi/go,josharian/go,cloudflare/go,cloudflare/go,golang/go,josharian/go,nawawi/go,akutz/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,akutz/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,golang/go,josharian/go,CAFxX/go,cloudflare/go,josharian/go,CAFxX/go