Allow modifying codegen-units from .cargo/config

[jdm]

There are a couple reasons why this could be useful:

• rather than forcing all developers to use a particular number of codegen units (unless they modify Cargo.toml, which is annoying in a VCS-tracked file), this allows overriding a suggested default with something appropriate to the current hardware
• this will allow me to work around codegen-units ruins debug symbols rust#39160 with much less friction while maintaining a sensible default for other Servo developers

[ehuss]

Setting codegen-units in config files can now be done via config-profiles (https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#config-profiles). The tracking issue for stabilization is at rust-lang/rust#48683.