Switch to the arm32 libunwind.a from the resource dir

Remove the -ldl because the Clang driver automatically adds -ldl when
it's needed for unwinding (dynamic bin/libs), and omits it from static
executables where the unwinder only needs libc.a.

Define the libunwind.a prebuilt for all architectures, even though
it's currently only used for arm32. We're planning to use it on all
architectures soon.

Bug: http://b/153025717
Test: checkbuild.py && run_tests.py
Change-Id: I03e74402ba1c42357691493015e7e39830ca8497