Enable LIBCXXABI_SILENT_TERMINATE for libc++abi.

Libc++abi's verbose terminate handler is only useful when exceptions are
enabled. Disabling it removes a few hundred thousand bytes from many C++
programs.

Makefile

@@ -105,6 +105,7 @@ build/libcxxabi.BUILT: build/libcxx.BUILT build/llvm.BUILT
 		-DCMAKE_C_COMPILER_WORKS=ON \
 		-DLIBCXXABI_ENABLE_EXCEPTIONS:BOOL=OFF \
 		-DLIBCXXABI_ENABLE_SHARED:BOOL=OFF \
+		-DLIBCXXABI_SILENT_TERMINATE:BOOL=ON \
 		-DLIBCXXABI_ENABLE_THREADS:BOOL=OFF \
 		-DLIBCXXABI_HAS_PTHREAD_API:BOOL=OFF \
 		-DLIBCXXABI_HAS_EXTERNAL_THREAD_API:BOOL=OFF \