[lld] [LLD][COFF] Require explicit specification of ARM64EC target (PR #116281)

[Daniel Paoliello via llvm-commits]

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@@ -46,6 +46,9 @@ static bool compatibleMachineType(COFFLinkerContext &ctx, MachineTypes mt) {
     return COFF::isArm64EC(mt) || mt == AMD64;
   case ARM64X:
     return COFF::isAnyArm64(mt) || mt == AMD64;
+  case IMAGE_FILE_MACHINE_UNKNOWN:
+    // The ARM64EC target must be explicitly specified and cannot be inferred.
+    return !isArm64EC(mt);
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dpaoliello wrote:

To match the lines above:
```suggestion
    return !COFF::isArm64EC(mt);
```

https://github.com/llvm/llvm-project/pull/116281