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)

hadrian/src/Settings/Builders/Configure.hs

@@ -27,7 +27,7 @@ configureBuilderArgs = do
                      -- https://gmplib.org/manual/Build-Options for
                      -- more detailed explanation of this configure
                      -- option.
-                     <> [ "--enable-alloca=malloc-reentrant" | targetArch == "wasm32" ]
+                     <> [ "--enable-alloca=malloc-notreentrant" | targetArch == "wasm32" ]
                      -- Enable PIC unless target is wasm32, in which
                      -- case we don't want libgmp.a to be bloated due
                      -- to PIC overhead.