Add overflow-checks=false and default target json.

Author: efenniht-furiosa

.cargo/config.toml

@@ -1,2 +1,5 @@
 [unstable]
 build-std = ["core", "compiler_builtins"]
+
+[build]
+target = "kernel-rs/riscv64gc-unknown-none-elfhf.json"

kernel-rs/Cargo.toml

@@ -26,12 +26,16 @@ scopeguard = { version = "1.1.0", default-features = false }
 arrayvec = { version = "0.5.1",  default-features = false }
 
 # Compile options for sysroot packages.
+# Cargo currently warns package `compiler_builtins` is not a dependency.
+# It is a bug, those options actually work well.
 [profile.dev.package.core]
 opt-level = 3
 debug-assertions = false
 codegen-units = 16
+overflow-checks = false
 
 [profile.dev.package.compiler_builtins]
 opt-level = 3
 debug-assertions = false
 codegen-units = 16
+overflow-checks = false