tests/legacy/build_constraints: Split Go asm test into own package (#3652)

The native "go build" tool compiles assembly in a Cgo package with
the C compiler, and compiles assembly in a pure Go package with the
Go assembler. The build_constraints test previously mixed Go
assembler files with Cgo files. This caused the native go test to
fail with:

package using cgo has Go assembly file asm_linux_amd64.s

Splitting this into two separate packages fixes this.

This problem was reported in:
#2006

This will fix failing tests in my attempt to fix that issue:
#3648

Author: evanj

tests/legacy/build_constraints/BUILD.bazel

@@ -16,10 +16,6 @@ go_library(
         # Check that tags are observed.
         "tag_l.go",
         "tag_unknown.go",
-        # Check that constraints apply to assembly files.
-        "asm_arm64.s",
-        "asm_linux_amd64.s",
-        "asm_unknown.s",
         # Check that constraints apply to cgo files.
         "cgo_linux.go",
         "cgo_unknown.go",

tests/legacy/build_constraints/build_constraints_test.go

@@ -25,23 +25,6 @@ func TestTag(t *testing.T) {
 	check(tag, t)
 }
 
-func asm() int
-
-func TestAsm(t *testing.T) {
-	got := asm()
-	var want int
-	if runtime.GOOS == "linux" {
-		want = 12
-	} else if runtime.GOARCH == "arm64" {
-		want = 75
-	} else {
-		want = 34
-	}
-	if got != want {
-		t.Errorf("got %d; want %d", got, want)
-	}
-}
-
 func TestCgoGo(t *testing.T) {
 	check(cgoGo, t)
 }

tests/legacy/build_constraints_go_asm/BUILD.bazel

@@ -0,0 +1,20 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+
+go_test(
+    name = "go_default_test",
+    size = "small",
+    srcs = ["build_constraints_go_asm_test.go"],
+    embed = [":go_default_library"],
+)
+
+go_library(
+    name = "go_default_library",
+    srcs = [
+        # Check that constraints apply to assembly files.
+        "asm_arm64.s",
+        "asm_linux_amd64.s",
+        "asm_unknown.s",
+    ],
+    cgo = False,
+    importpath = "github.com/bazelbuild/rules_go/tests/build_constraints_go_asm",
+)

tests/legacy/build_constraints/asm_arm64.s renamed to tests/legacy/build_constraints_go_asm/asm_arm64.s

@@ -0,0 +1,23 @@
+package build_constraints_go_asm
+
+import (
+	"runtime"
+	"testing"
+)
+
+func asm() int
+
+func TestAsm(t *testing.T) {
+	got := asm()
+	var want int
+	if runtime.GOOS == "linux" {
+		want = 12
+	} else if runtime.GOARCH == "arm64" {
+		want = 75
+	} else {
+		want = 34
+	}
+	if got != want {
+		t.Errorf("got %d; want %d", got, want)
+	}
+}