[CMake] Fix libc++abi arm build w/o libunwind.

[CMake] Fix libc++abi arm build w/o libunwind.

This commit fixes libc++abi build when LLVM unwinder (libunwind_llvm) is
not enabled.

This commit fixes the problem by removing "LLVM_NATIVE_ARCH MATCHES ARM"
from CMakeLists.txt so that LIBCXXABI_USE_LLVM_UNWINDER will only be
defined when LLVM unwinder is enabled.

We need LIBCXXABI_USE_LLVM_UNWINDER becase there is a subtle difference
between the unwinder from libgcc and the one from libunwind_llvm.  For
the unwinder from libgcc, we have to initialize register r12 with the
address of _Unwind_Control_Block; otherwise,
_Unwind_GetLanguageSpecificData() and _Unwind_GetRegionStart() won't
work properly.  Consequently, there is an extra _Unwind_SetGR() when
LLVM unwinder is disabled.  Check cxa_personality.cpp for details.

llvm-svn: 286759