cmd/compile, etc: use name offset in method tables

cmd/compile, etc: use name offset in method tables

Introduce and start using nameOff for two encoded names. This pair
of changes is best done together because the linker's method decoder
expects the method layouts to match.

Precursor to converting all existing name and *string fields to
nameOff.

linux/amd64:
	cmd/go:  -45KB (0.5%)
	jujud:  -389KB (0.6%)

linux/amd64 PIE:
	cmd/go: -170KB (1.4%)
	jujud:  -1.5MB (1.8%)

For #6853.

Change-Id: Ia044423f010fb987ce070b94c46a16fc78666ff6
Reviewed-on: https://go-review.googlesource.com/21396
Reviewed-by: Ian Lance Taylor <iant@golang.org>