Fix compile user for RV32

user/Xargo.toml

@@ -0,0 +1,6 @@
+[dependencies]
+alloc = {}
+
+[dependencies.compiler_builtins]
+features = ["mem"]
+stage = 1

user/riscv32-ucore.json

@@ -9,7 +9,7 @@
   "cpu": "generic-rv32",
   "features": "+m",
   "max-atomic-width": "32",
-  "linker": "ld.lld",
+  "linker": "riscv32-unknown-elf-ld",
   "linker-flavor": "ld",
   "executables": true,
   "panic-strategy": "abort",

user/ucore-ulib/src/lib.rs

@@ -4,6 +4,9 @@
 #![feature(panic_implementation)]
 #![feature(panic_info_message)]
 #![feature(linkage)]
+#![feature(compiler_builtins_lib)]
+
+extern crate compiler_builtins;
 
 #[macro_use]
 pub mod syscall;