cmd/internal/gc, cmd/yacc: implement "expecting" syntax error messages

cmd/internal/gc, cmd/yacc: implement "expecting" syntax error messages

Bison includes suggestions about what tokens are expected in the
current state when there's only four or fewer of them.  For example:

  syntax error: unexpected literal 2.01, expecting semicolon or newline or }

This CL adds the same functionality to cmd/yacc, which fully restores
the previous error message behavior from Go 1.4.

Updates #9968.

Change-Id: I2c1a1677c6d829a829d812c05e8813aa8829d09c
Reviewed-on: https://go-review.googlesource.com/8494
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>