wasm: set the stack at the start of linear memory

This makes sure that a stack overflow will cause a "memory access out of
bounds" error instead of a corruption of a global variable.

Here is more background on a very similar stack overflow protection:
https://blog.japaric.io/stack-overflow-protection/

targets/wasm.json

@@ -14,6 +14,7 @@
 	"ldflags": [
 		"--allow-undefined",
 		"--no-threads",
+		"--stack-first",
 		"--export-all"
 	],
 	"emulator":      ["node", "targets/wasm_exec.js"]