text/template: avoid a global map to help the linker's deadcode elimination

Fixes #36021
Updates #2559
Updates #26775

Change-Id: I2e6708691311035b63866f25d5b4b3977a118290
Reviewed-on: https://go-review.googlesource.com/c/go/+/210284


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>