fuzzgen: Increase allowed stack space

[afonso360]

👋 Hey,

The motivation for this PR is the encoding error that @uweigand found in #5786. He had previously run fuzzgen on s390x and that sort of error should have been found. But it wasn't because we never generate large offsets in our stack accesses.

This PR allows larger stack slots that let us explore these larger offsets and hopefully more errors like this.

I've picked 8192 somewhat arbitrarily, it's the next largest power of two after 4096 which i suspect might be the more troublesome value, although this probably depends on the ISA.

With 8192 we have a max stack usage per function of 8 * 8192 = 65536. And with #5765 we generate at most 8 functions, so worst case 8 * 8 * 8192 = 524288 which still seems fairly reasonable.

Opening this as a draft because it has already found issues on the AArch64 backend! (#5824)