testsuite/android: build test executables with the android ndk directly

Summary:
This teaches the test makefiles about the Android NDK, so we are able to
run the tests without first going through the make_standalone_toolchain
script. The motivation for this is the ability to run both libc++ and
libstdc++ tests together, which previously was not possible because
make_standalone_toolchain bakes in the STL to use during toolchain
creation time. The support for this is not present yet -- this change
only make sure we don't regress for existing funcionality (gcc w/
libstdc++). Clang and libc++ support will be added later.

I've checked that the mips android targets compile after this change,
but I have no way of checking whether this breaks anything. If you are
reading this and it broke you, let me know.

Reviewers: tberghammer, danalbert

Subscribers: srhines, lldb-commits

Differential Revision: https://reviews.llvm.org/D30410

llvm-svn: 296869