[libc] Move the internal extern "C" symbols inside the namespace (#98232)

This ensures that these symbols inherit the namespace visibility.

Author: petrhosek

libc/src/__support/OSUtil/baremetal/exit.cpp

@@ -8,11 +8,11 @@
 
 #include "src/__support/OSUtil/exit.h"
 
+namespace LIBC_NAMESPACE::internal {
+
 // This is intended to be provided by the vendor.
 extern "C" [[noreturn]] void __llvm_libc_exit(int status);
 
-namespace LIBC_NAMESPACE::internal {
-
 [[noreturn]] void exit(int status) { __llvm_libc_exit(status); }
 
 } // namespace LIBC_NAMESPACE::internal

libc/src/__support/OSUtil/baremetal/io.cpp

@@ -10,15 +10,15 @@
 
 #include "src/__support/CPP/string_view.h"
 
+namespace LIBC_NAMESPACE {
+
 // This is intended to be provided by the vendor.
 
 extern struct __llvm_libc_stdin __llvm_libc_stdin;
 extern "C" ssize_t __llvm_libc_stdin_read(void *cookie, char *buf, size_t size);
 
 extern "C" void __llvm_libc_log_write(const char *msg, size_t len);
 
-namespace LIBC_NAMESPACE {
-
 ssize_t read_from_stdin(char *buf, size_t size) {
   return __llvm_libc_stdin_read(reinterpret_cast<void *>(&__llvm_libc_stdin),
                                 buf, size);

libc/src/stdlib/exit.cpp

@@ -10,10 +10,10 @@
 #include "src/__support/OSUtil/exit.h"
 #include "src/__support/common.h"
 
-extern "C" void __cxa_finalize(void *);
-
 namespace LIBC_NAMESPACE {
 
+extern "C" void __cxa_finalize(void *);
+
 [[noreturn]] LLVM_LIBC_FUNCTION(void, exit, (int status)) {
   __cxa_finalize(nullptr);
   internal::exit(status);

libc/startup/baremetal/fini.cpp

@@ -9,13 +9,13 @@
 #include <stddef.h>
 #include <stdint.h>
 
+namespace LIBC_NAMESPACE {
+
 extern "C" {
 extern uintptr_t __fini_array_start[];
 extern uintptr_t __fini_array_end[];
 }
 
-namespace LIBC_NAMESPACE {
-
 using FiniCallback = void(void);
 
 extern "C" void __libc_fini_array(void) {

libc/startup/baremetal/init.cpp

@@ -9,15 +9,15 @@
 #include <stddef.h>
 #include <stdint.h>
 
+namespace LIBC_NAMESPACE {
+
 extern "C" {
 extern uintptr_t __preinit_array_start[];
 extern uintptr_t __preinit_array_end[];
 extern uintptr_t __init_array_start[];
 extern uintptr_t __init_array_end[];
 }
 
-namespace LIBC_NAMESPACE {
-
 using InitCallback = void(void);
 
 extern "C" void __libc_init_array(void) {