Disable tcmalloc on i386 since google-perftools isn't available there on Ubuntu

Along with !4 (merged) this would allow the Debian jpeg-xl package to autosync to Ubuntu.

Ubuntu only build select packages on i386 (roughly what is needed for wine and steam etc to run so that legacy apps can be run on an otherwise amd64 install). I was unable to get google-perftools to successfully build there when I attempted it last year.

Debian hasn't gone as far as Ubuntu, but Debian is planning for i386 to not be a directly installable architecture in the next major release (Debian 13 "Trixie"). Therefore, I think it's ok for the i386 package to be built without the tcmalloc performance optimization.