Fix AVX512 nomalloc issues in trsm.

The AVX512 trsm kernels end up always trying to allocate via malloc. Notably, EIGEN_STACK_ALLOCATION_LIMIT is 0 for AVX512 by default (and within tests), so we cannot use ei_declare_aligned_stack_constructed_variable either. The only solution is to disable the AVX512 kernels if malloc is disabled.

This fixes the nomalloc tests.