Bump mono to 2018-08-watchos branch

Commit list for mono/mono:

* mono/mono@b6c818b204d [arm64_32] use emulate OPs so it works with bitcode compiled code for armv7k (xamarin#12443)
* mono/mono@204f917c972 [sdks] Add a XCODE32_DIR make variable and use it to compile the 32 bit cross compilers. (xamarin#12055)
* mono/mono@70a207adaa7 [ios] initial watch64_32 support (xamarin#11691)
* mono/mono@287b5598f93 [mini] rename s/__mono_ilp32__/MONO_ARCH_ILP32 (xamarin#11652)
* mono/mono@9939c1a6560 [System]Fix race condition in `SystemDependencyProvider.Initialize()`. xamarin#12538.
* mono/mono@b0c4390d608 [runtime] Don't probe aot cache on android
* mono/mono@725ba2a2523 Fix mono/mono#12461.
* mono/mono@1dee62626d4 [2018-08 Android,AOT] Support dumping AOT offsets with Android NDK r17+ (xamarin#12454) (xamarin#12465)
* mono/mono@146c8191fb2 [x86] Set the LLVM data layout.
* mono/mono@5d3077c7bcb [arm] Set the LLVM data layout, without it, opt seems to make incorrect optimizations.
* mono/mono@5266e6a8f10 [2018-08][debugger][backport] Fix crash when there is a generic struct with a field that is an enumerator (xamarin#12410)
* mono/mono@0d988bc696e Bump API snapshot submodule
* mono/mono@e6285d015b9 Bump corefx

Diff: mono/mono@51c4f45...b6c818b

.gitmodules

@@ -8,7 +8,7 @@
 [submodule "external/mono"]
     path = external/mono
     url = ../../mono/mono.git
-    branch = 2018-08
+    branch = 2018-08-watchos
 [submodule "external/opentk"]
     path = external/opentk
     url = ../../mono/opentk.git