[android][armv7] Mark let_properties_opts.swift as XFAIL.

Looks like Android ARMv7 is not inline storeBytes so the test were
failing. Mark the test as XFAIL and open a bug in the tracker to keep
the CI green for the moment.

Author: drodriguez

test/SILOptimizer/let_properties_opts.swift

@@ -3,6 +3,9 @@
 
 // REQUIRES: optimized_stdlib
 
+// See https://bugs.swift.org/browse/SR-12370
+// XFAIL: OS=linux-androideabi && CPU=armv7
+
 // Test propagation of non-static let properties with compile-time constant values.
 
 // TODO: Once this optimization can remove the propagated fileprivate/internal let properties or