Move PIE from submodule into MAMBO repository

[IgWod]

This is something still very much to be discussed, but I think the current sentiment is to merge PIE and MAMBO together into a single repo. This has a number of benefits:

1. Simplifies process of merging new instructions into MAMBO
2. Reduces a possibility of MAMBO and PIE getting out of sync
3. Simplifies repo management and access rights

I don't see any potential downside of such move, but I'm very much opened for comments.

Once that's done we need to port following PRs into this repo:

1. Arm v8.2 pie#8
2. A64: Add support for ARMv9.0 pie#9

[ijan1]

Might be worth putting in the path to https://github.com/beehive-lab/pie in a commit message for archival purposes, otherwise LGTM.