cmd/dist: unleash bootstrap optimization for windows

This belongs to a series of clean-up changes (see below) for cmd/dist.
This is change (3).

These changes include:
(1)  apply minor fixes
(2)  restore behavior of branchtag
(3)  unleash bootstrap optimization for windows
(4)  use standard generated code header
(5)  remove trivial variables + functions
(6)  move functions for the better
(7)  simplify code segments
(8)  use bytes.Buffer for code generation
(9)  rename variables + functions
(10) remove doc.go

Change-Id: I7612bbc3939e6fca3bee4b8e92c528178dd46cd7
Reviewed-on: https://go-review.googlesource.com/61023Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/dist/buildtool.go

@@ -190,7 +190,7 @@ func bootstrapBuildTools() {
 	xprintf("\n")
 }
 
-var ssaRewriteFileSubstring = filepath.ToSlash("src/cmd/compile/internal/ssa/rewrite")
+var ssaRewriteFileSubstring = filepath.FromSlash("src/cmd/compile/internal/ssa/rewrite")
 
 // isUnneededSSARewriteFile reports whether srcFile is a
 // src/cmd/compile/internal/ssa/rewriteARCHNAME.go file for an