Add cold attribute to one time construction APIs

[Aditya K via Gcc-patches]

This would help compiler optimize local static objects.

```
commit e2f299679ddf56a6d6d71ea9d589cd76b2ca107b
Author: Aditya Kumar <1894981+hiradi...@users.noreply.github.com>
Date:   Thu Aug 13 09:41:34 2020 -0700

    Add cold attribute to one time construction APIs
    
    __cxa_guard_acquire is used for only one purpose,
    namely guarding local static variable initialization,
    and since that purpose is definitionally cold, it should be attributed as 
cold.
    Similarly for __cxa_guard_release and __cxa_guard_abort

diff --git a/libstdc++-v3/include/bits/c++config 
b/libstdc++-v3/include/bits/c++config
index b1fad59d4..359e955a7 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -39,20 +39,24 @@
 // Macros for various attributes.
 //   _GLIBCXX_PURE
 //   _GLIBCXX_CONST
 //   _GLIBCXX_NORETURN
 //   _GLIBCXX_NOTHROW
 //   _GLIBCXX_VISIBILITY
 #ifndef _GLIBCXX_PURE
 # define _GLIBCXX_PURE __attribute__ ((__pure__))
 #endif
 
+#ifndef _GLIBCXX_COLD
+# define _GLIBCXX_COLD __attribute__ ((cold))
+#endif
+
 #ifndef _GLIBCXX_CONST
 # define _GLIBCXX_CONST __attribute__ ((__const__))
 #endif
 
 #ifndef _GLIBCXX_NORETURN
 # define _GLIBCXX_NORETURN __attribute__ ((__noreturn__))
 #endif
 
 // See below for C++
 #ifndef _GLIBCXX_NOTHROW
diff --git a/libstdc++-v3/libsupc++/cxxabi.h b/libstdc++-v3/libsupc++/cxxabi.h
index 000713ecd..24c1366e2 100644
--- a/libstdc++-v3/libsupc++/cxxabi.h
+++ b/libstdc++-v3/libsupc++/cxxabi.h
@@ -108,27 +108,27 @@ namespace __cxxabiv1
   __cxa_vec_delete2(void* __array_address, size_t __element_size,
                    size_t __padding_size, __cxa_cdtor_type __destructor,
                    void (*__dealloc) (void*));
 
   void
   __cxa_vec_delete3(void* __array_address, size_t __element_size,
                    size_t __padding_size, __cxa_cdtor_type __destructor,
                    void (*__dealloc) (void*, size_t));
 
   int
-  __cxa_guard_acquire(__guard*);
+  __cxa_guard_acquire(__guard*) _GLIBCXX_COLD;
 
   void
-  __cxa_guard_release(__guard*) _GLIBCXX_NOTHROW;
+  __cxa_guard_release(__guard*) _GLIBCXX_NOTHROW _GLIBCXX_COLD;
 
   void
-  __cxa_guard_abort(__guard*) _GLIBCXX_NOTHROW;
+  __cxa_guard_abort(__guard*) _GLIBCXX_NOTHROW _GLIBCXX_COLD;
 
   // DSO destruction.
   int
   __cxa_atexit(void (*)(void*), void*, void*) _GLIBCXX_NOTHROW;
 
   void
   __cxa_finalize(void*);
 
   // TLS destruction.
   int
```