Add arm64ec platform expression

[autoantwort]

@kobykahane Or do you prefer that arm64 also matches arm64ec

[kobykahane]

@kobykahane Or do you prefer that arm64 also matches arm64ec

Ideally arm64 and arm64ec should be separate, because in my use case, I would like to build the capstone port as follows:

• For all triplets, with the 'diet' feature.
• For the x86 and arm64ec triplets, with the 'x86' feature.
• For the arm64 and arm64ec triplets, with the 'arm64' feature.
  (In other words, on ARM64EC I would like both the x86 and ARM64 disassemblers, because both types of code might be present).
  Looking something like:

    {
      "name": "capstone",
      "default-features": false,
      "features": [
        {
          "name": "arm64",
          "platform": "arm64,arm64ec"
        },
        "diet",
        {
          "name": "x86",
          "platform": "x86,arm64ec"
        }
      ]
    },

[BillyONeal]

Thanks!