Move zkVM constants into sys::env_consts

I missed this in rust-lang#139868. Its `mod` declaration was removed, but the
contents were not moved.

Author: thaliaarchi

std/src/sys/env_consts.rs

@@ -389,6 +389,17 @@ pub mod os {
     pub const EXE_EXTENSION: &str = "exe";
 }
 
+#[cfg(target_os = "zkvm")]
+pub mod os {
+    pub const FAMILY: &str = "";
+    pub const OS: &str = "";
+    pub const DLL_PREFIX: &str = "";
+    pub const DLL_SUFFIX: &str = ".elf";
+    pub const DLL_EXTENSION: &str = "elf";
+    pub const EXE_SUFFIX: &str = ".elf";
+    pub const EXE_EXTENSION: &str = "elf";
+}
+
 // The fallback when none of the other gates match.
 #[else]
 pub mod os {