cmd/internal/objabi: extract shared functionality from obj

Now only cmd/asm and cmd/compile depend on cmd/internal/obj. Changing
the assembler backends no longer requires reinstalling cmd/link or
cmd/addr2line.

There's also now one canonical definition of the object file format in
cmd/internal/objabi/doc.go, with a warning to update all three
implementations.

objabi is still something of a grab bag of unrelated code (e.g., flag
and environment variable handling probably belong in a separate "tool"
package), but this is still progress.

Fixes #15165.
Fixes #20026.

Change-Id: Ic4b92fac7d0d35438e0d20c9579aad4085c5534c
Reviewed-on: https://go-review.googlesource.com/40972
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

1 files changed, 5 insertions, 2 deletions

diff --git a/src/cmd/compile/internal/ssa/rewritedec64.go b/src/cmd/compile/internal/ssa/rewritedec64.go
index 605128c767..4b9b3783e2 100644
--- a/src/cmd/compile/internal/ssa/rewritedec64.go
+++ b/src/cmd/compile/internal/ssa/rewritedec64.go
@@ -5,9 +5,12 @@ package ssa
 
 import "math"
 import "cmd/internal/obj"
+import "cmd/internal/objabi"
+
+var _ = math.MinInt8  // in case not otherwise used
+var _ = obj.ANOP      // in case not otherwise used
+var _ = objabi.GOROOT // in case not otherwise used
 
-var _ = math.MinInt8 // in case not otherwise used
-var _ = obj.ANOP     // in case not otherwise used
 func rewriteValuedec64(v *Value) bool {
 	switch v.Op {
 	case OpAdd64: