Update build-std to support a boolean or array.

[Alexhuszagh]

Checklist

• I've looked through the issues and pull requests for similar request
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Describe your request

When using -Z build-std, cargo also enables you to pass various different flags which will specify the exact components to build, rather than all components. This can be useful if std isn't supported, but core and alloc are. Therefore, build-std could be provided as an array or bool (we shouldn't allow a table), and then passed transparent to -Z build-std.

[target.x86_64-unknown-dragonfly]
build-std = true

[target.thumbv6m-none-eabi]
build-std = ["core", "alloc"]

Describe why this would be a good inclusion for cross

Allows more comprehensive use of build-std, without breaking old behavior.