Allocate the argv buffers with alloca instead of malloc.

[sunfishcode]

This way, if an application doesn't otherwise use malloc, they don't need to link in malloc.

Idea from https://github.com/CraneStation/wasi-libc/pull/118#discussion_r337596386!

One thing I'm somewhat concerned by is the lack of error reporting in alloca. Perhaps that's a broader problem that we'll want to address anyway?

[sunfishcode]

I am considering waiting until __stack_limit is available (patch here), followed by a way to check for stack overflow in clang, before enabling this.