Implement memfd with a raw syscall for compat with old glibc

Author: saethlin

src/sys/memfd.rs

@@ -49,7 +49,8 @@ pub fn memfd_create(name: &CStr, flags: MemFdCreateFlag) -> Result<OwnedFd> {
                 any(
                     target_os = "freebsd",
                     // If the OS is Linux, gnu and musl expose a memfd_create symbol but not uclibc
-                    target_env = "gnu",
+                    // *but* the glibc symbol was only added in 2.27, so for compatability with
+                    // older systems we use the raw syscall on target_env = "gnu".
                     target_env = "musl",
                 )))]
             {