cmd/compile: make gc debug flags collector a struct

gc debug flags are currently stored in a 256-long array, that is then
addressed using the ASCII numeric value of the flag itself (a quirk
inherited from the old C compiler). It is also a little wasteful,
since we only define 16 flags, and the other 240 array elements are
always empty.

This change makes Debug a struct, which also provides static checking
that we're not referencing flags that does not exist.

Change-Id: I2f0dfef2529325514b3398cf78635543cdf48fe0
Reviewed-on: https://go-review.googlesource.com/c/go/+/263539


Trust: Alberto Donizetti <alb.donizetti@gmail.com>
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>