Make it easier to run test variants.

This adds all the permutations for tests as make targets of the form
test-libcxx-(host|target)-(clang|gcc)-(32|64).

This also changes the host tests to use the Android build system (like
I had done with https://android-review.googlesource.com/#/c/111924/).
This probably should have been a separate patch, but I got carried
away (and wanted to make sure this new approach would work for both),
and now they're non-trivial to split.

Change-Id: Ie99caf6c3ff21c833408f99d37299d966ee7bc94
diff --git a/test.mk b/test.mk
index 31e8ed2..0837d11 100644
--- a/test.mk
+++ b/test.mk
@@ -1,6 +1,4 @@
 .NOTPARALLEL:
 default:
-	cp $(ANDROID_BUILD_TOP)/external/libcxx/test/lit.$(LIT_MODE).cfg \
-	   $(ANDROID_BUILD_TOP)/external/libcxx/test/lit.site.cfg
-	-python $(LIT) -sv $(ANDROID_BUILD_TOP)/external/libcxx/test
-	rm $(ANDROID_BUILD_TOP)/external/libcxx/test/lit.site.cfg
+	-python $(LIT) -sv --param android_mode=$(LIT_MODE) \
+        $(ANDROID_BUILD_TOP)/external/libcxx/test