Add a recursivity guard for GPA::allocate(). This means that any
recursive allocations will fall back to the supporting allocator. In future
patches, we will introduce stack trace collection support. The unwinder will be
provided by the supporting allocator, and we can't guarantee they don't call
malloc() (e.g. backtrace() on posix may call dlopen(), which may call malloc().
Furthermore, this patch packs the new TLS reentrancy guard into a thread local
struct, so that TLS variables should be hopefully not fall across cache lines.