[libc] Add baremetal abort

[lntue]

Can you run clang-format before committing?

[abrachet]

In D147795#4251739, @lntue wrote:

Can you run clang-format before committing?

Done in commit

libc/src/stdlib/baremetal/CMakeLists.txt

+add_entrypoint_object(
+  abort
+  SRCS
+    abort.cpp
+  HDRS
+    ../abort.h
+)

libc/src/stdlib/baremetal/abort.cpp

+//===-- Implementation of abort -------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/common.h"
+
+#include "src/stdlib/abort.h"
+
+namespace __llvm_libc {
+
+LLVM_LIBC_FUNCTION(void, abort, ()) { __builtin_trap(); }
+
+} // namespace __llvm_libc