gdb/unittests: PR28413, suppress warnings generated by Gnulib

gdb/unittests: PR28413, suppress warnings generated by Gnulib

Gnulib generates a warning if the system version of certain functions
are used (to redirect the developer to use Gnulib version).  It caused a
compiler error when...

-   Compiled with Clang
-   -Werror is specified (by default)
-   C++ standard used by Clang is before C++17 (by default as of 15.0.0)
    when this unit test is activated.

This issue is raised as PR28413.

However, previous proposal to fix this issue (a "fix" to Gnulib):
<https://lists.gnu.org/archive/html/bug-gnulib/2021-10/msg00003.html>
was rejected because it ruins the intent of Gnulib warnings.

So, we need a Binutils/GDB-side solution.

This commit tries to address this issue on the GDB side.  We have
"include/diagnostics.h" to disable certain warnings only when necessary.

This commit suppresses the Gnulib warnings by surrounding entire #include
block with DIAGNOSTIC_IGNORE_USER_DEFINED_WARNINGS to disable Gnulib-
generated warnings on all standard C++ header files.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28413
Approved-By: Simon Marchi <simon.marchi@efficios.com>
Change-Id: Ieeb5a31a6902808d4c7263a2868ae19a35e0ccaa