Issue Description:

As far as I recall, libunwind isn't available as part of the Android NDK, so the part of the Swift runtime that normally emits backtraces is ifdef'd out when compiling for Android. As a result, I assume the test/Runtime/backtrace.swift would almost certainly fail on Android – it should be disabled.

Prerequisites:

• An Ubuntu 15.10 development environment capable of building Swift from source.

• A physical Android device.

Helpful skills:
Basic knowledge of C++ would help, but isn't necessary.

Starting point:

1. Follow the instructions in docs/Android.md to build and run the test suite on a physical Android device. Confirm this test still fails.

2. Backtraces appear to be disabled on Android:

   swift/stdlib/public/runtime/Errors.cpp

   Lines 17 to 21 in 91ffcd7

   	#if defined(__CYGWIN__) || defined(__ANDROID__) || defined(_MSC_VER)
   	# define SWIFT_SUPPORTS_BACKTRACE_REPORTING 0
   	#else
   	# define SWIFT_SUPPORTS_BACKTRACE_REPORTING 1
   	#endif

   – Confirm this is still the case by reading through how that macro is used in that file.

3. Read through test/Runtime/backtrace.swift, confirm that it does indeed test backtraces. If it does, add an unsupported: or requires: flag to the top of test/Runtime/backtrace.swift to keep it from running on platforms that don't support backtraces.