cmd/compile: remove -installsuffix flag

Obsoleted by -importcfg.

cmd/link has a similar flag, but it seems to still be needed at least
for misc/cgo/testshared.TestGopathShlib. I can't immediately tell why
(has something to do with finding .so files), but it doesn't appear to
possibly affect cmd/compile.

Updates #51225.

Change-Id: I80c6aef860bd162c010ad4a1a4f532b400cf901c
Reviewed-on: https://go-review.googlesource.com/c/go/+/415236
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/compile/doc.go

@@ -68,9 +68,6 @@ Flags:
 	-importcfg file
 		Read import configuration from file.
 		In the file, set importmap, packagefile to specify import resolution.
-	-installsuffix suffix
-		Look for packages in $GOROOT/pkg/$GOOS_$GOARCH_suffix
-		instead of $GOROOT/pkg/$GOOS_$GOARCH.
 	-l
 		Disable inlining.
 	-lang version

src/cmd/compile/internal/base/flag.go

@@ -100,7 +100,6 @@ type CmdFlags struct {
 	GenDwarfInl        int          "help:\"generate DWARF inline info records\"" // 0=disabled, 1=funcs, 2=funcs+formals/locals
 	GoVersion          string       "help:\"required version of the runtime\""
 	ImportCfg          func(string) "help:\"read import configuration from `file`\""
-	InstallSuffix      string       "help:\"set pkg directory `suffix`\""
 	JSON               string       "help:\"version,file for JSON compiler/optimizer detail output\""
 	Lang               string       "help:\"Go language version source code expects\""
 	LinkObj            string       "help:\"write linker-specific object to `file`\""

src/cmd/compile/internal/noder/import.go

@@ -102,9 +102,7 @@ func openPackage(path string) (*os.File, error) {
 
 	if buildcfg.GOROOT != "" {
 		suffix := ""
-		if base.Flag.InstallSuffix != "" {
+		if base.Flag.Race {
-			suffix = "_" + base.Flag.InstallSuffix
-		} else if base.Flag.Race {
 			suffix = "_race"
 		} else if base.Flag.MSan {
 			suffix = "_msan"

src/cmd/go/internal/work/gc.go

@@ -128,9 +128,6 @@ func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg
 	if extFiles == 0 {
 		defaultGcFlags = append(defaultGcFlags, "-complete")
 	}
-	if cfg.BuildContext.InstallSuffix != "" {
-		defaultGcFlags = append(defaultGcFlags, "-installsuffix", cfg.BuildContext.InstallSuffix)
-	}
 	if a.buildID != "" {
 		defaultGcFlags = append(defaultGcFlags, "-buildid", a.buildID)
 	}