Don't force the "mem" feature for -none targets.

WHY ON EARTH WOULD COMPILER-BUILTINS FORCE MEMSET/MEMCMP/MEMMOVE/MEMCPY
TO BE IMLPEMENTED FOR ALL KERNEL TARGETS, AND BEING IMPOSSIBLE TO OPT
OUT OF WITHOUT PATCHING RUSTC?

build.rs

@@ -27,7 +27,6 @@ fn main() {
     // provide them.
     if (target.contains("wasm32") && !target.contains("wasi"))
         || (target.contains("sgx") && target.contains("fortanix"))
-        || target.contains("-none")
         || target.contains("nvptx")
     {
         println!("cargo:rustc-cfg=feature=\"mem\"");