go/build: use static maps rather than an init func

go/build is one of the packages that contributes the most towards
cmd/go's init cost, which adds up to any call to the tool.

One piece of low-hanging fruit is knownOS and knownArch,
maps which are filled via an init func from a space-separated list.
Using GODEBUG=inittrace=1, we can get three samples:

	init go/build @0.36 ms, 0.024 ms clock, 6568 bytes, 74 allocs
	init go/build @0.33 ms, 0.025 ms clock, 6888 bytes, 76 allocs
	init go/build @0.36 ms, 0.025 ms clock, 6728 bytes, 75 allocs

After using a static map instead, we see an improvement:

	init go/build @0.33 ms, 0.018 ms clock, 5096 bytes, 69 allocs
	init go/build @0.36 ms, 0.021 ms clock, 5096 bytes, 69 allocs
	init go/build @0.33 ms, 0.019 ms clock, 5096 bytes, 69 allocs

The speedup isn't huge, but it helps, and also reduces allocs.
One can also imagine that the compiler may get better with static,
read-only maps in the future, whereas the init func will likely always
have a linear cost and extra allocations.

Change-Id: I430212bad03d25358d2cc7b1eab4536ad88d05a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/390274


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Trust: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>