[release-branch.go1] cmd/gc: Suggest *T in error for x.(T) if it would work.

««« backport 33a62d86ca2f
cmd/gc: Suggest *T in error for x.(T) if it would work.

Accomplished by synchronizing the formatting of conversion errors between typecheck.c and subr.c

Fixes #3984.

R=golang-dev, remyoudompheng, rsc
CC=golang-dev
https://golang.org/cl/6500064

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