Add code coverage uploads for non-amd64 architectures

[codefromthecrypt]

Is your feature request related to a problem? Please describe.
https://app.codecov.io/gh/tetratelabs/wazero doesn't include all main sources, rather just amd64

We should have all sources. There are notably more for arm64, but also a few for neither (risc-v)

Describe the solution you'd like
I'd like codecov to present a merged view of all sources, which are materialized by emulated arm64 and risc-v also uploading reports.

Describe alternatives you've considered
not doing anything

Additional context
#596 (comment)

[codefromthecrypt]

note: the GitHub action for codecov doesn't yet support non-amd64 codecov/uploader#523

[codefromthecrypt]

TL;DR; let's use the normal bash script until non-amd64 works on the action, or they drop the bash script

if they really delete the bash script before fixing non-amd64 a complicated workaround would be to have a fan-in upload stage. basically run all the coverage in a matrix and upload artifacts with the coverage.txt. block on that, download all the coverage.txt into a different directory or path and then use codecov/codecov-action@v2 action to send them at the same time.

The ideal way out is to wait for non-amd64 to work and use the bash script meanwhile.

[philippgille]

The referenced PR for the codecov uploader is now closed since 2 days and they link to arch-specific uploader binaries, so we could give it a try.

One question:

Currently the coverage only runs as part of the test_amd64 job here.

Is your plan to

1. Rename that to be arch independent and add other architectures to the matrix?
2. Or add arm64 and riscv64 jobs next to it?
3. Or add coverage tracking to the test_scratch job, which already has all three architectures in [its matrix](https://github.com/tetratelabs/wazero/blob/9138b6f722ca855
   c8f498df6749f302a8a4e9ffa/.github/workflows/commit.yaml#L107-L114)?