perf: tests: Adapt mmap-basic.c for riscv

riscv now supports mmaping hardware counters to userspace so adapt the test
to run on this architecture.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Ian Rogers <irogers@google.com>