Restore parallel codegen and incr builds

rust-lang/rust#47074 is fixed now.

Author: berkus

Cargo.toml

@@ -26,13 +26,6 @@ rlibc = "1.0.0"
 bitflags = "1.0.1"
 multiboot2 = "0.3.2"
 
-# by japaric:
-# disable both incremental compilation and parallel codegen to reduce the chances of running into
-# rust-lang/rust#47074
-[profile.dev]
-codegen-units = 1
-incremental = false
-
 [profile.release]
 debug = true
 lto = true