libstdc++: Disable all emergency EH pool code if obj-count == 0

libstdc++: Disable all emergency EH pool code if obj-count == 0

For a zero-sized static pool we can completely elide all code for the EH
pool.

We no longer need to adjust the static buffer size to ensure at least
one free_entry can be created in it, because we no longer use a static
buffer at all if obj_count == 0. If the buffer exists, obj_count >= 1
and the buffer will be much larger than sizeof(free_entry).

libstdc++-v3/ChangeLog:

	* libsupc++/eh_alloc.cc [USE_POOL]: New macro.
	[!USE_POOL] (__gnu_cxx::__freeres, pool): Do not define.
	[_GLIBCXX_EH_POOL_STATIC] (pool::arena): Do not use std::max.
	(__cxxabiv1::__cxa_allocate_exception) [!USE_POOL]: Do not use
	pool.
	(__cxxabiv1::__cxa_free_exception) [!USE_POOL]: Likewise.
	(__cxxabiv1::__cxa_allocate_dependent_exception) [!USE_POOL]:
	Likewise.
	(__cxxabiv1::__cxa_free_dependent_exception) [!USE_POOL]:
	Likewise.