Make the mmap64 implementation Linux-specific.

`mmap64` is only defined on Linux, so move the implementation
to Linux-specific code.

src/tools/miri/src/shims/unix/foreign_items.rs

@@ -266,12 +266,6 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 let ptr = this.mmap(addr, length, prot, flags, fd, offset)?;
                 this.write_scalar(ptr, dest)?;
             }
-            "mmap64" => {
-                let [addr, length, prot, flags, fd, offset] = this.check_shim(abi, Abi::C {unwind: false}, link_name, args)?;
-                let offset = this.read_scalar(offset)?.to_i64()?;
-                let ptr = this.mmap(addr, length, prot, flags, fd, offset.into())?;
-                this.write_scalar(ptr, dest)?;
-            }
             "munmap" => {
                 let [addr, length] = this.check_shim(abi, Abi::C {unwind: false}, link_name, args)?;
                 let result = this.munmap(addr, length)?;

src/tools/miri/src/shims/unix/linux/foreign_items.rs

@@ -9,6 +9,7 @@ use shims::unix::fs::EvalContextExt as _;
 use shims::unix::linux::fd::EvalContextExt as _;
 use shims::unix::linux::mem::EvalContextExt as _;
 use shims::unix::linux::sync::futex;
+use shims::unix::mem::EvalContextExt as _;
 use shims::unix::sync::EvalContextExt as _;
 use shims::unix::thread::EvalContextExt as _;
 
@@ -217,6 +218,14 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 this.write_null(dest)?;
             }
 
+            "mmap64" => {
+                let [addr, length, prot, flags, fd, offset] =
+                    this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
+                let offset = this.read_scalar(offset)?.to_i64()?;
+                let ptr = this.mmap(addr, length, prot, flags, fd, offset.into())?;
+                this.write_scalar(ptr, dest)?;
+            }
+
             _ => return Ok(EmulateForeignItemResult::NotSupported),
         };