cc-wrapper: only set -march when platform.gcc.arch is specified
cc-wrapper: only set -march when platform.gcc.arch is specified

We want to make sure this value is explicitly set. Infering it for
every arch leads to annoying failures like:

https://hydra.nixos.org/build/92583832/

Perhaps we can enable it in the future with some smarter handling of
cc-wrapper.sh.

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