target/arm: Handle SME-only CPUs in sve_vqm1_for_el_sm()

In sve_vqm1_for_el_sm(), we implicitly assume that the CPU has SVE:
if called with sm == false for non-streaming mode, we try to return a
vector length from svq_vq.  This hits the "assert(sm)" at the bettom
of the function in an SME-only CPU where sve_vq.map is zero.

Add code to handle the "SME-only CPU not in streaming mode" case: we
report an effective VL of 128 bits, which is what the architecture
rule R_KXKNK says should be used when SVE instructions are disabled
or trapped but floating point instructions are enabled.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20260202133353.2231685-7-peter.maydell@linaro.org