Allow overriding the magic numbers

[Daniel-Abrecht]

I needed a larger stack for one of my projects, so I fixed all those magic numbers and made it so it can use __stack_pointer if exported and so on.

I open this pull request out of courtesy, but I don't expect it to be merged, for I won't accept googles CLA. The only CLA I will ever accept is the DCO, because CLAs generally do not benefit anyone but the one asking for them. Open source projects already have a license everyone needs to honor, and which gives everyone involved equal rights to their contributions. Normalizing having to give anyone additional rights or guarantees, no matter how benign they may seam, is not in anyones interest.

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