ADD: improve Makefile in tests/, add explicit arch flags

[T-K-233]

changelog:omit

Minor change. Added the complete variable definition of the riscv64-unknown-elf- toolchain. Added the ARCH and ABI flags definitions.

The default compilation flow is not affected.

Related PRs / Issues:

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

Contributor Checklist:

• Did you set main as the base branch?
• Is this PR's title suitable for inclusion in the changelog and have you added a changelog:<topic> label?
• Did you state the type-of-change/impact?
• Did you delete any extraneous prints/debugging code?
• Did you mark the PR with a changelog: label?
• (If applicable) Did you update the conda .conda-lock.yml file if you updated the conda requirements file?
• (If applicable) Did you add documentation for the feature?
• (If applicable) Did you add a test demonstrating the PR?

• (If applicable) Did you mark the PR as Please Backport?

[jerryz123]

I'll give you write permissions to chipyard. For future PRs, can you push the branch directly into Chipyard? Then the CI can run, and we don't have to create copies of PRs.