[Brett-D]

Problem

With RX_MARCH=znver2 (Steam Deck, Zen 2: AVX2, FMA and more), nx2/build.py printed build: this CPU has no AVX2: the game is built for znver2 (a little slower). The message assumed any level other than x86-64-v3 was the CPU check's fallback.

Change

this CPU has no AVX2 is now printed only when host_march() chose the level. An explicit RX_MARCH other than x86-64-v3 is reported as build: the game is built for <march> (RX_MARCH). Flags are unchanged.

Testing

tests/test_cpu_target.py gains a test that runs the actual selection block from build.py with mocked RX_MARCH and host_march() values. It fails on main (znver2 reported as "no AVX2") and passes on Windows and Linux.

[hckrman101]

Looks good. The message now says "no AVX2" only when host_march() made the choice, and an explicit RX_MARCH is reported as such. The flags are untouched. The empty-string RX_MARCH case behaves the same as before (falls through to host_march()), and the test's four cases cover both branches, including the silent ones. All CI checks are green and the PR is mergeable.

Note on #2383 (single-precision FMA/FRECPS/FRSQRTS fused on x86-64-v3): it only touches the comment above this block in nx2/build.py, so the two don't conflict. The interaction is that an explicit RX_MARCH now gets a truthful message, which matters more once the fast path depends on the level. An RX_MARCH=znver2 build has FMA but is not x86-64-v3, so #2383 should key on the feature (__FMA__) rather than on the string x86-64-v3. That is for #2383, not this PR.

Nit: the test slices build.py by text and execs it. That is fine here and matches how other tests in the repo work, but it will need updating if the block is reshaped.