libstdc++: Allow emergency EH alloc pool size to be tuned [PR68606]

Implement a long-standing request to support tuning the size of the
emergency buffer for allocating exceptions after malloc fails, or to
disable that buffer entirely.

It's now possible to disable the dynamic allocation of the buffer and
use a fixed-size static buffer, via --enable-libstdcxx-static-eh-pool.
This is a built-time choice that is baked into libstdc++ and so affects
all code linked against that build of libstdc++.

The size of the pool can be set by --with-libstdcxx-eh-pool-obj-count=N
which is measured in units of sizeof(void*) not bytes. A given exception
type such as std::system_error depends on the target, so giving a size
in bytes wouldn't be portable across 16/32/64-bit targets.

When libstdc++ is configured to use a dynamic buffer, the size of that
buffer can now be tuned at runtime by setting the GLIBCXX_TUNABLES
environment variable (c.f. PR libstdc++/88264). The number of exceptions
to reserve space for is controlled by the "glibcxx.eh_pool.obj_count"
and "glibcxx.eh_pool.obj_size" tunables. The pool will be sized to be
able to allocate obj_count exceptions of size obj_size*sizeof(void*) and
obj_count "dependent" exceptions rethrown by std::rethrow_exception.

With the ability to tune the buffer size, we can reduce the default pool
size on 32-bit and 16-bit targets. Most users never need to throw 1kB
exceptions in parallel from hundreds of threads after malloc is OOM. The
users who do need that can use the tunables to select larger sizes.

The old defaults can be chosen at runtime by setting GLIBCXX_TUNABLES
to:
64-bit: glibcxx.eh_pool.obj_count=64:glibcxx.eh_pool.obj_size=112
32-bit: glibcxx.eh_pool.obj_count=32:glibcxx.eh_pool.obj_size=104

Or approximated by configuring with:
64-bit: --with-libstdcxx-eh-pool-obj-count=252
32-bit: --with-libstdcxx-eh-pool-obj-count=94

libstdc++-v3/ChangeLog:

	PR libstdc++/68606
	* Makefile.in: Regenerate.
	* acinclude.m4 (GLIBCXX_EMERGENCY_EH_ALLOC): New macro.
	* configure: Regenerate.
	* configure.ac: Use GLIBCXX_EMERGENCY_EH_ALLOC.
	* crossconfig.m4: Check for secure_getenv.
	* doc/Makefile.in: Regenerate.
	* doc/xml/manual/configure.xml: Document new configure options.
	* doc/xml/manual/evolution.xml: Document addition of tunables.
	* doc/xml/manual/using_exceptions.xml: Document emergency
	buffer and tunables.
	* doc/html/*: Regenerate.
	* include/Makefile.in: Regenerate.
	* libsupc++/Makefile.am: Use EH_POOL_FLAGS.
	* libsupc++/Makefile.in: Regenerate.
	* libsupc++/eh_alloc.cc (EMERGENCY_OBJ_SIZE): Define in units
	of sizeof(void*) not including the ABI's exception header.
	(EMERGENCY_OBJ_COUNT): Define as target-independent calculation
	based on word size.
	(MAX_OBJ_COUNT): Define macro for upper limit on pool size.
	(pool) [_GLIBCXX_EH_POOL_STATIC]: Use fixed-size buffer.
	(pool::buffer_size_in_bytes): New static member function.
	(pool::pool): Parse GLIBCXX_TUNABLES environment variable to set
	pool size at runtime.
	(pool::in_pool): Use std::less<void*> for total order.
	(__freeres) [_GLIBCXX_EH_POOL_STATIC]: Do nothing.
	(__cxa_free_exception, __cxa_free_dependent_exception): Add
	[[unlikely]] attributes.
	* po/Makefile.in: Regenerate.
	* python/Makefile.in: Regenerate.
	* src/Makefile.in: Regenerate.
	* src/c++11/Makefile.in: Regenerate.
	* src/c++17/Makefile.in: Regenerate.
	* src/c++20/Makefile.in: Regenerate.
	* src/c++98/Makefile.in: Regenerate.
	* src/filesystem/Makefile.in: Regenerate.
	* src/libbacktrace/Makefile.in: Regenerate.
	* testsuite/Makefile.in: Regenerate.