hadrian: build in-tree GMP with malloc-notreentrant on wasm32

This patch makes hadrian build in-tree GMP with the
--enable-alloca=malloc-notreentrant configure option. We will only
need malloc-reentrant when we have threaded RTS and SMP support on
wasm32, which will take some time to happen, before which we should
use malloc-notreentrant to avoid undesired runtime overhead.

(cherry picked from commit 06277d56)