gnulib: update to 776af40e0

This fixes PR27184, a failure to compile gdb due to
cdefs.h being out of sync with glibc on ppc64le targets
which are compiled with -mabi=ieeelongdouble and glibc
2.32.

Likewise, update usage of _GL_ATTRIBUTE_FORMAT_PRINTF to
_GL_ATTRIBUTE_FORMAT_PRINTF_STANDARD.

Likewise, disable newly added rpl_free gnulib api in
gdbserver support libraries.

Likewise, undefine read/write macros before redefining them
on mingw targets.

Likewise, wrap C++ usage of free with GNULIB_NAMESPACE namespace
as needed.

Change-Id: I86517613c0d8ac8f5ea45bbc4ebe2b54a3aef29f

gnulib/import/float.in.h

@@ -1,6 +1,6 @@
 /* A correct <float.h>.
 
-   Copyright (C) 2007-2020 Free Software Foundation, Inc.
+   Copyright (C) 2007-2021 Free Software Foundation, Inc.
 
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -93,11 +93,14 @@
      extern const long double LDBL_MAX;
 
    Unfortunately, this is not a constant expression.  */
+# if !GNULIB_defined_long_double_union
 union gl_long_double_union
   {
     struct { unsigned int lo; unsigned int hi; unsigned int exponent; } xd;
     long double ld;
   };
+#  define GNULIB_defined_long_double_union 1
+# endif
 extern const union gl_long_double_union gl_LDBL_MAX;
 # define LDBL_MAX (gl_LDBL_MAX.ld)
 /* Minimum e such that 10^e is in the range of normalized numbers.  */
@@ -146,11 +149,14 @@ extern const union gl_long_double_union gl_LDBL_MAX;
 
    Unfortunately, this is not a constant expression, and the latter expression
    does not work well when GCC is optimizing..  */
+# if !GNULIB_defined_long_double_union
 union gl_long_double_union
   {
     struct { double hi; double lo; } dd;
     long double ld;
   };
+#  define GNULIB_defined_long_double_union 1
+# endif
 extern const union gl_long_double_union gl_LDBL_MAX;
 # define LDBL_MAX (gl_LDBL_MAX.ld)
 #endif