Add basic reentrancy-checking support, and allow arena_new to reenter.

This checks whether or not we're reentrant using thread-local data, and, if we
are, moves certain internal allocations to use arena 0 (which should be properly
initialized after bootstrapping).

The immediate thing this allows is spinning up threads in arena_new, which will
enable spinning up background threads there.

test/stress/microbench.c

@@ -156,7 +156,7 @@ TEST_END
 
 int
 main(void) {
-	return test(
+	return test_no_reentrancy(
 	    test_malloc_vs_mallocx,
 	    test_free_vs_dallocx,
 	    test_dallocx_vs_sdallocx,