Add support for multiple targets

[haata]

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Motivation

I have a larger embedded project that supports multiple boards (each with their own crate).
The build process uses cargo-make and supports multiple targets during a top-level build.

Currently I need the following targets for a successful build:

• thumbv6m-none-eabi
• thumbv7em-none-eabi
• thumbv7em-none-eabihf

Along with x86_64-unknown-linux-gnu for unit tests.

It would be useful to specify all the needed targets as a list, just like the components field.

Workflow example

      - uses: actions-rs/toolchain@v1
        with:
          toolchain: nightly
          targets: thumbv6m-none-eabi, thumbv7em-none-eabi, thumbv7em-none-eabihf
          override: true

I have no preference of target vs. targets. targets is probably easier for backward compatibility.