cmd/internal/obj: add IsAsm flag

This allows more exciting changes to compiler-generated assembly
language that might not be correct for tricky hand-crafted
assembly (e.g., nop padding breaking tables of call or branch
instructions).

Updates #35881

Change-Id: I842b811796076c160180a364564f2844604df3fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/229708
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/asm/main.go

@@ -41,6 +41,7 @@ func main() {
 	ctxt.Flag_dynlink = *flags.Dynlink
 	ctxt.Flag_shared = *flags.Shared || *flags.Dynlink
 	ctxt.Flag_go115newobj = *flags.Go115Newobj
+	ctxt.IsAsm = true
 	switch *flags.Spectre {
 	default:
 		log.Printf("unknown setting -spectre=%s", *flags.Spectre)

src/cmd/internal/obj/link.go

@@ -675,7 +675,8 @@ type Link struct {
 
 	InParallel           bool // parallel backend phase in effect
 	Framepointer_enabled bool
-	UseBASEntries        bool // Use Base Address Selection Entries in location lists and PC ranges
+	UseBASEntries        bool // use Base Address Selection Entries in location lists and PC ranges
+	IsAsm                bool // is the source assembly language, which may contain surprising idioms (e.g., call tables)
 
 	// state for writing objects
 	Text []*LSym