Add libc++abi darwin file group

This commit adds `libc++abi_darwin` file group because `libc++.1.dylib`
depends on `libc++abi.1.dylib`.

Bug: 111579848
Test: prebuilts/clang-tools/build-prebuilts.sh
Test: out/soong/dist/bin/header-abi-dumper --help
Change-Id: I453166991bc64974e5978af4200e899912b5f4d9
1 file changed