Handle the 'enabled' property in bp2build

Also fix some bugs pertaining to configurable attribute handling of bool
attributes and label sttributes, so that they may support values across
multiple different axes at the same time.

Test: unit tests for bp2build
Test: mixed_droid

Change-Id: I411efcfddf02d55dbc0775962068a11348a8bb2c
diff --git a/bazel/configurability.go b/bazel/configurability.go
index 1993f76..7355ac7 100644
--- a/bazel/configurability.go
+++ b/bazel/configurability.go
@@ -109,6 +109,21 @@
 		osArchWindowsX86_64:        "//build/bazel/platforms/os_arch:windows_x86_64",
 		ConditionsDefaultConfigKey: ConditionsDefaultSelectKey, // The default condition of an os select map.
 	}
+
+	// Map where keys are OsType names, and values are slices containing the archs
+	// that that OS supports.
+	// These definitions copied from arch.go.
+	// TODO(cparsons): Source from arch.go; this task is nontrivial, as it currently results
+	// in a cyclic dependency.
+	osToArchMap = map[string][]string{
+		osAndroid:     {archArm, archArm64, archX86, archX86_64},
+		osLinux:       {archX86, archX86_64},
+		osLinuxMusl:   {archX86, archX86_64},
+		osDarwin:      {archArm64, archX86_64},
+		osLinuxBionic: {archArm64, archX86_64},
+		// TODO(cparsons): According to arch.go, this should contain archArm, archArm64, as well.
+		osWindows: {archX86, archX86_64},
+	}
 )
 
 // basic configuration types
@@ -122,6 +137,10 @@
 	productVariables
 )
 
+func osArchString(os string, arch string) string {
+	return fmt.Sprintf("%s_%s", os, arch)
+}
+
 func (ct configurationType) String() string {
 	return map[configurationType]string{
 		noConfig:         "no_config",