use a single codegen-unit with the dev profile

rust-lang/rust#44853 changed the default number of codegen units from 1 to 32 for the dev profile.
Unfortunately this broke our dev builds so we are reverting the change in the Cargo.toml.

Author: japaric

Cargo.toml

@@ -6,7 +6,7 @@ keywords = ["arm", "cortex-m", "template"]
 license = "MIT OR Apache-2.0"
 name = "cortex-m-quickstart"
 repository = "https://github.com/japaric/cortex-m-quickstart"
-version = "0.2.1"
+version = "0.2.2"
 
 [dependencies]
 cortex-m = "0.3.0"
@@ -16,6 +16,9 @@ cortex-m-semihosting = "0.2.0"
 features = ["abort-on-panic"]
 version = "0.3.3"
 
+[profile.dev]
+codegen-units = 1
+
 [profile.release]
 debug = true
 lto = true