Add a zkASM architecture

This is a very initial take on this if only to be able to specify
`zkasm-unknown-unknown` target in tooling that uses `target-lexicon`.
Some of the details are not exactly 100% correct in the current state of
the machine (e.g. pointer width is 256 bits), but this will potentially
change soon…

src/targets.rs

@@ -47,6 +47,10 @@ pub enum Architecture {
     X86_64h,
     XTensa,
     Clever(CleverArchitecture),
+    /// A software machine that produces zero-knowledge proofs of the execution.
+    ///
+    /// See https://wiki.polygon.technology/docs/category/zk-assembly/
+    ZkAsm,
 }
 
 #[cfg_attr(feature = "rust_1_40", non_exhaustive)]
@@ -855,7 +859,8 @@ impl Architecture {
             | S390x
             | Sparc
             | Sparc64
-            | Sparcv9 => Ok(Endianness::Big),
+            | Sparcv9
+            | ZkAsm => Ok(Endianness::Big),
         }
 
     }
@@ -896,6 +901,9 @@ impl Architecture {
             | LoongArch64
             | Wasm64
             | Clever(_) => Ok(PointerWidth::U64),
+            // Actually 256-bit, as are the rest of registers…
+            // But definitely can't address that much memory.
+            ZkAsm  => Ok(PointerWidth::U64),
         }
     }
 
@@ -943,6 +951,7 @@ impl Architecture {
             X86_64h => Cow::Borrowed("x86_64h"),
             XTensa => Cow::Borrowed("xtensa"),
             Clever(ver) => ver.into_str(),
+            ZkAsm => Cow::Borrowed("zkasm"),
         }
     }
 }
@@ -1218,6 +1227,7 @@ impl FromStr for Architecture {
             "x86_64" => X86_64,
             "x86_64h" => X86_64h,
             "xtensa" => XTensa,
+            "zkasm" => ZkAsm,
             _ => {
                 if let Ok(arm) = ArmArchitecture::from_str(s) {
                     Arm(arm)
@@ -1709,6 +1719,7 @@ mod tests {
             "x86_64-wrs-vxworks",
             "xtensa-esp32-espidf",
             "clever-unknown-elf",
+            "zkasm-unknown-unknown",
         ];
 
         for target in targets.iter() {