cv32a6 benchmarks - Cross-compiler options

[pcotret]

I was playing with a cv32a6 variant. In riscv-tests, while ISA tests are compiled for both 32-bit and 64-bit, benchmarks will only be compiled for 64-bits when running ci/build-riscv-tests.sh

I'm trying to find an elegant to do it without breaking everything: it should be a modification in riscv-tests Makefile.