[cmake] Make lldb build with the android ndk toolchain file

Summary:
The NDK cmake toolchain file defines CMAKE_SYSTEM_NAME=Android, so switch the
build to use that. I have also updated the in-tree toolchain file to do that
(instead of defining __ANDROID_NDK__), so it can still be used to build.
After migrating the last bits of non-toolchainy bits out of the in-tree
toolchain, I intend to delete it.

Reviewers: tberghammer, danalbert

Subscribers: srhines, mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D28775

llvm-svn: 292212