Cranelift: Add LibCall::Memcmp

The comment says the enum is "likely to grow" and the function's been in libc since C89, so hopefully this is ok.

I'd like to use it for emitting things like array equality.

cranelift/codegen/src/ir/libcall.rs

@@ -56,6 +56,8 @@ pub enum LibCall {
     Memset,
     /// libc.memmove
     Memmove,
+    /// libc.memcmp
+    Memcmp,
 
     /// Elf __tls_get_addr
     ElfTlsGetAddr,
@@ -92,6 +94,7 @@ impl FromStr for LibCall {
             "Memcpy" => Ok(Self::Memcpy),
             "Memset" => Ok(Self::Memset),
             "Memmove" => Ok(Self::Memmove),
+            "Memcmp" => Ok(Self::Memcmp),
 
             "ElfTlsGetAddr" => Ok(Self::ElfTlsGetAddr),
             _ => Err(()),
@@ -157,6 +160,7 @@ impl LibCall {
             Memcpy,
             Memset,
             Memmove,
+            Memcmp,
             ElfTlsGetAddr,
         ]
     }
@@ -201,4 +205,11 @@ mod tests {
     fn parsing() {
         assert_eq!("FloorF32".parse(), Ok(LibCall::FloorF32));
     }
+
+    #[test]
+    fn all_libcalls_to_from_string() {
+        for &libcall in LibCall::all_libcalls() {
+            assert_eq!(libcall.to_string().parse(), Ok(libcall));
+        }
+    }
 }