Enable compilation for specific target

By exposing the target information through `CompilerConfig`,
compiler(only LLVM at the moment) could create a machine with
different CPU feature flags other than current host, which makes it
capable to "cross compile" to some degree.

Update #959

lib/runtime-core/src/backend.rs

@@ -129,6 +129,11 @@ pub struct CompilerConfig {
     pub enforce_stack_check: bool,
     pub track_state: bool,
     pub features: Features,
+
+    // target info used by LLVM
+    pub triple: Option<String>,
+    pub cpu_name: Option<String>,
+    pub cpu_features: Option<String>,
 }
 
 pub trait Compiler {