wasm: avoid miscompile with ThinLTO

I found that when I enable ThinLTO, a miscompilation triggers that had
been hidden all the time previously. The bug appears to happen as
follows:

 1. TinyGo generates a function with a runtime.trackPointer call, but
    without an alloca (or the alloca gets optimized away).
 2. LLVM sees that no alloca needs to be kept alive across the
    runtime.trackPointer call, and therefore it adds the 'tail' flag.
    One of the effects of this flag is that it makes it undefined
    behavior to keep allocas alive across the call (which is still safe
    at that point).
 3. The GC lowering pass adds a stack slot alloca and converts
    runtime.trackPointer calls into alloca stores.

The last step triggers the bug: the compiler inserts an alloca where
there was none before but that's not valid as long as the 'tail' flag is
present.

This patch fixes the bug in a somewhat dirty way, by always creating a
dummy alloca so that LLVM won't do the optimization in step 2 (and
possibly other optimizations that rely on there being no alloca
instruction).

src/runtime/gc_stack_portable.go

@@ -53,7 +53,7 @@ func markStack() {
 // trackPointer is a stub function call inserted by the compiler during IR
 // construction. Calls to it are later replaced with regular stack bookkeeping
 // code.
-func trackPointer(ptr unsafe.Pointer)
+func trackPointer(ptr, alloca unsafe.Pointer)
 
 // swapStackChain swaps the stack chain.
 // This is called from internal/task when switching goroutines.