Introduce crt_static target option in config.toml

This controls the value of the crt-static feature used when building the
standard library for a target, as well as the compiler itself when that
target is the host.

src/bootstrap/lib.rs

@@ -656,6 +656,12 @@ impl Build {
         base
     }
 
+    /// Returns if this target should statically link the C runtime, if specified
+    fn crt_static(&self, target: Interned<String>) -> Option<bool> {
+        self.config.target_config.get(&target)
+            .and_then(|t| t.crt_static)
+    }
+
     /// Returns the "musl root" for this `target`, if defined
     fn musl_root(&self, target: Interned<String>) -> Option<&Path> {
         self.config.target_config.get(&target)