interp: fix the logic to skip source files based on OS or CPU arch

For example, on architecture GOARCH=amd64, a file named `foobar_amd64.go` would be skipped instead of being read and parsed. The function `skipFile` is fixed and missing tests are added.

interp/build.go

@@ -147,10 +147,22 @@ func skipFile(ctx *build.Context, p string, skipTest bool) bool {
 	}
 	a := strings.Split(p[i+1:], "_")
 	last := len(a) - 1
-	if last1 := last - 1; last1 >= 0 && a[last1] == ctx.GOOS && a[last] == ctx.GOARCH {
-		return false
+	if last-1 >= 0 {
+		switch x, y := a[last-1], a[last]; {
+		case x == ctx.GOOS:
+			if knownArch[y] {
+				return y != ctx.GOARCH
+			}
+			return false
+		case knownOs[x] && knownArch[y]:
+			return true
+		case knownArch[y] && y != ctx.GOARCH:
+			return true
+		default:
+			return false
+		}
 	}
-	if s := a[last]; s != ctx.GOOS && s != ctx.GOARCH && knownOs[s] || knownArch[s] {
+	if x := a[last]; knownOs[x] && x != ctx.GOOS || knownArch[x] && x != ctx.GOARCH {
 		return true
 	}
 	return false

interp/build_test.go

@@ -51,7 +51,7 @@ func TestBuildTag(t *testing.T) {
 	}
 }
 
-func TestBuildFile(t *testing.T) {
+func TestSkipFile(t *testing.T) {
 	// Assume a specific OS, arch and go pattern no matter the real underlying system
 	ctx := build.Context{
 		GOARCH: "amd64",
@@ -65,10 +65,18 @@ func TestBuildFile(t *testing.T) {
 		{"bar_linux.go", false},
 		{"bar_maix.go", false},
 		{"bar_mlinux.go", false},
+
 		{"bar_aix_foo.go", false},
+		{"bar_linux_foo.go", false},
+		{"bar_foo_amd64.go", false},
+		{"bar_foo_arm.go", true},
+
 		{"bar_aix_s390x.go", true},
 		{"bar_aix_amd64.go", true},
 		{"bar_linux_arm.go", true},
+
+		{"bar_amd64.go", false},
+		{"bar_arm.go", true},
 	}
 
 	for _, test := range tests {