Is there a way to set a `--cfg` flag only for specific target in .cargo/config.toml?

Question

I'd like to pass a custom --cfg flag to rustc via Cargo. Normally, I can do this by putting this in .cargo/config for a given crate:

[build]
rustflags = "--cfg procmacro2_semver_exempt"

However, in one of my crates, I am using a custom compilation target, and that seems to break rustflags;

[build]
target = "avr-atmega32u4.json"
rustflags = "--cfg procmacro2_semver_exempt"

[unstable]
build-std = ["core"]

[env]
AVR_CPU_FREQUENCY_HZ = "16_000_000"

In this case, the config flag is not passed to rustc. I've tried moving rustflags into a target-specific section:

[target.avr-atmega32u4.json]
rustflags = "--cfg procmacro2_semver_exempt"

but this doesn't seem to have any effect either.