[backend] Add x86_64/machine-ir.h and impls

This also adds scripts to generate the header and impls for
machine-ir.h. Other parts of the implementation are done in the
x86_64/code*.cc files.

Bug: 291127204
Test: mm and berberis_host_tests
Change-Id: I0a367143bd931268c85534d389c147c884b6e802
Merged-In: If84e3ca4a5404bd19d52a4e7461907f65c6e132d
11 files changed