Disable memfd_create tests on ART host tests.

memfd_create only supports bionic-based platforms (see code below). I
plan to investigate whether it _should_ support host, and whether
it needs to be added to libcore's virtualdeviceknownfailures.txt,
separately but for now this CL fixes the urgent issue of the test
breaking the ART host tests.

Test: Treehugger
Change-Id: Ibbbcaffd7abd1784f9e5ee51393f1e83ba6b6291
diff --git a/tools/libcore_failures.txt b/tools/libcore_failures.txt
index 98e8e19..5d4f3ef 100644
--- a/tools/libcore_failures.txt
+++ b/tools/libcore_failures.txt
@@ -9,6 +9,14 @@
 
 [
 {
+  description: "Os.memfd_create() only supports bionic-based platforms.",
+  result: EXEC_FAILED,
+  modes: [host],
+  names: ["libcore.android.system.OsTest#testMemfdCreate",
+          "libcore.android.system.OsTest#testMemfdCreateErrno",
+          "libcore.android.system.OsTest#testMemfdCreateFlags"]
+},
+{
   description: "Assert.java differences between vogar and junit.",
   result: EXEC_FAILED,
   modes: [host],