[Backtracing] Rename _Backtracing to Runtime.

Move the backtracing code into a new Runtime module.  This means renaming
the Swift Runtime's CMake target because otherwise there will be a name
clash.

rdar://124913332

stdlib/public/RuntimeModule/modules/OS/Linux.h

@@ -0,0 +1,42 @@
+//===--- Linux.h - Linux specifics ------------------------------*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+//  Linux specific includes and declarations.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_BACKTRACING_LINUX_H
+#define SWIFT_BACKTRACING_LINUX_H
+#ifdef __linux__
+
+#define _GNU_SOURCE
+#include <sys/uio.h>
+#include <ucontext.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+ssize_t process_vm_readv(pid_t pid,
+                         const struct iovec *local_iov,
+                         unsigned long liovcnt,
+                         const struct iovec *remote_iov,
+                         unsigned long riovcnt,
+                         unsigned long flags);
+
+#ifdef __cplusplus
+} // extern "C"
+#endif
+
+#endif // __linux__
+#endif // SWIFT_BACKTRACING_LINUX_H
+