test(activation): skip fp4 silu_and_mul_quant on non-gfx950 archs

[zjin-lcf]

FP4 (e2m1) output relies on CDNA4-only MFMA and is compiled in only for gfx950/gfx1250 (guarded by #if defined(__Float4_e2m1fn_x2) in csrc/kernels/activation_kernels.cu). On gfx942 (MI300A/MI300X) the fp4 branch is compiled out, so the dispatch hits AITER_CHECK(false) and aborts the whole test process. Gate the fp4 case on the runtime gfx (matching the mxfp4 skip idiom in test_moe_ep.py) so it is skipped cleanly, letting the remaining activation tests run.

[github-actions]

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