Add the rust std as a custom sysroot

.cargo/config.toml

@@ -3,10 +3,9 @@ target = "riscv64.json"
 
 [unstable]
 json-target-spec = true
-build-std = ["core", "compiler_builtins", "alloc"]
-build-std-features = ["compiler-builtins-mem"]
 
 [target.riscv64]
 rustflags = [
   "-C", "link-arg=-Tilm.ld",
+  "--sysroot", "sysroot"
 ]