x86: add Herd7-based Litmus parser

[MattWindsor91]

Work in the #97 area will make it necessary to be able to parse x86 litmus tests directly. This means that we'll have to implement a parser for x86 litmus tests using the Herd7 dialect.

This is almost certainly going to be a significant pain in the posterior, mitigated by two things:

• we can reuse bits of the ATT dialect parser;
• we can reuse bits of Herd's parser (within reason; we want to implement a superset of its parsing abilities).

In future, we might be able to pitch an expanded form of the Herd parser back to the Herd project.

[MattWindsor91]

Closing this as we've dropped X86 assembly support.