amdgcn: Implement proper locks: Fix 'newlib/libc/sys/amdgcn/include/sys/lock.h' for C++

As of commit 7dd4eb1db9
"amdgcn: Implement proper locks", we get, by the thousands, for C++ code:

    In file included from [...]/newlib/libc/include/stdio.h:60,
                     from [...]:
    [...]/newlib/libc/include/sys/reent.h:911:1: error: expected declaration before '}' token

newlib/libc/sys/amdgcn/include/sys/lock.h

@@ -4,6 +4,10 @@
 #include <newlib.h>
 #include <_ansi.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 typedef unsigned int _LOCK_T;
 typedef unsigned int _LOCK_RECURSIVE_T;