This implements P2546R5 (Debugging Support), including the P2810R4
(is_debugger_present is_replaceable) changes, allowing
std::is_debugger_present to be replaced by the program.

It would be good to provide a macOS definition of is_debugger_present as
per https://developer.apple.com/library/archive/qa/qa1361/_index.html
but that isn't included in this change.

The src/c++26/debugging.cc file defines a global volatile int which can
be set by debuggers to indicate when they are attached and detached from
a running process. This allows std::is_debugger_present() to give a
reliable answer, and additionally allows a debugger to choose how
std::breakpoint() should behave. Setting the global to a positive value
will cause std::breakpoint() to use that value as an argument to
std::raise, so debuggers that prefer SIGABRT for breakpoints can select
that. By default std::breakpoint() will use a platform-specific action
such as the INT3 instruction on x86, or GCC's __builtin_trap().

On Linux the std::is_debugger_present() function checks whether the
process is being traced by a process named "gdb", "gdbserver" or
"lldb-server", to try to avoid interpreting other tracing processes
(such as strace) as a debugger. There have been comments suggesting this
isn't desirable and that std::is_debugger_present() should just return
true for any tracing process (which is the case for non-Linux targets
that support the ptrace system call).

libstdc++-v3/ChangeLog:

	PR libstdc++/119670
	* acinclude.m4 (GLIBCXX_CHECK_DEBUGGING): Check for facilities
	needed by <debugging>.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Use GLIBCXX_CHECK_DEBUGGING.
	* include/Makefile.am: Add new header.
	* include/Makefile.in: Regenerate.
	* include/bits/version.def (debugging): Add.
	* include/bits/version.h: Regenerate.
	* include/precompiled/stdc++.h: Add new header.
	* src/c++26/Makefile.am: Add new file.
	* src/c++26/Makefile.in: Regenerate.
	* include/std/debugging: New file.
	* src/c++26/debugging.cc: New file.
	* testsuite/19_diagnostics/debugging/breakpoint.cc: New test.
	* testsuite/19_diagnostics/debugging/breakpoint_if_debugging.cc:
	New test.
	* testsuite/19_diagnostics/debugging/is_debugger_present.cc: New
	test.
	* testsuite/19_diagnostics/debugging/is_debugger_present-2.cc:
	New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>