Fork crate `riscv` as a submodule

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "crate/riscv"]
+	path = crate/riscv
+	url = https://github.com/riscv-and-rust-and-decaf/riscv.git

Cargo.toml

@@ -39,7 +39,7 @@ bit-allocator = { path = "crate/bit-allocator" }
 ucore-memory = { path = "crate/memory" }
 
 [target.riscv32i-unknown-none.dependencies]
-riscv = { git = "https://github.com/riscv-rust/riscv" }
+riscv = { path = "crate/riscv" }
 bbl = { path = "crate/bbl" }
 
 [build-dependencies]

crate/riscv

@@ -0,0 +1 @@
+Subproject commit 87bcdd8bab940f37c01ea34b62f2fb2d0317ca88