[Bug libstdc++/88264] Support glibc-style tunables for

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88264

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I forgot to list the growth factor for pool resource too (currently hardcoded
to 2 in __pool_resource::_Pool::replenish())