Use TARGET instead of PREFIX in /tests Makefile(s)

[raghav-g13]

Issue: In /tests, when building libgloss locally, i.e., with BUILD_LIBGLOSS=1, libgloss.mk expects the variable TARGET to be defined. It uses --host=$(TARGET) for GCC cross-compilation. Currently, this errors out.

Fix: Set TARGET in tests/Makefile. Replace the use of PREFIX with TARGET so we only have a single source of truth.

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]

Can you rebase this on master with Abe's new CI changes? Then the tests should all run ... and faster too.