[libc++abi] Use from-scratch testing configs for libc++abi by default Like we have been doing for libc++ for a while now, start using from-scratch testing configurations for libc++abi. As a fly-by fix, remove the LIBCXXABI_NO_TIMER macro, which was defined but never used. Differential Revision: https://reviews.llvm.org/D125242 GitOrigin-RevId: 851bfc07c86e7f84ee38d918cb4b6b49a47434f1