Switch NDK libc++ from prebuilts/ndk to prebuilts/clang

Remove old NDK libc++ Soong modules, and use libunwind instead of
ndk_libunwind.

Bug: http://b/332594828
Test: m nothing --no-skip-soong-tests
Test: m blueprint_tests
Change-Id: I6763a4dbba660240e47a4a2eb179382c0a0a9840