Add hooking functionality

This allows us to hook chosen functions and do interesting things there (in
particular: reentrancy checking).

src/hooks.c

@@ -0,0 +1,12 @@
+#include "jemalloc/internal/jemalloc_internal.h"
+
+/*
+ * The hooks are a little bit screwy -- they're not genuinely exported in the
+ * sense that we want them available to end-users, but we do want them visible
+ * from outside the generated library, so that we can use them in test code.
+ */
+JEMALLOC_EXPORT
+void (*hooks_arena_new_hook)() = NULL;
+
+JEMALLOC_EXPORT
+void (*hooks_libc_hook)() = NULL;