Make FirstTarget treat HostCross separately from Host

Use Target.Os and Target.HostCross as the key in FirstTarget so that
it returns a separate target for host and host cross architectures.
This is useful when host and host cross are both linux_musl, but
host cross is an independenct architecture like arm64.

Bug: 236052820
Test: cross compile linux_musl-arm64
Change-Id: If75790001afe9d0f9d4d8166f207847851812297

android/arch.go

@@ -1843,20 +1843,23 @@ func getCommonTargets(targets []Target) []Target {
 }
 
 // FirstTarget takes a list of Targets and a list of multilib values and returns a list of Targets
-// that contains zero or one Target for each OsType, selecting the one that matches the earliest
-// filter.
+// that contains zero or one Target for each OsType and HostCross, selecting the one that matches
+// the earliest filter.
 func FirstTarget(targets []Target, filters ...string) []Target {
 	// find the first target from each OS
 	var ret []Target
-	hasHost := false
-	set := make(map[OsType]bool)
+	type osHostCross struct {
+		os        OsType
+		hostCross bool
+	}
+	set := make(map[osHostCross]bool)
 
 	for _, filter := range filters {
 		buildTargets := filterMultilibTargets(targets, filter)
 		for _, t := range buildTargets {
-			if _, found := set[t.Os]; !found {
-				hasHost = hasHost || (t.Os.Class == Host)
-				set[t.Os] = true
+			key := osHostCross{t.Os, t.HostCross}
+			if _, found := set[key]; !found {
+				set[key] = true
 				ret = append(ret, t)
 			}
 		}