lmbench: use base_libdir instead of hardcoded /lib

Fix the below QA warning when building 64bit target:
| WARNING: QA Issue: lmbench: Files/directories were installed but not
| shipped
| /usr/lib
| /usr/lib/libmbench.a

Signed-off-by: Ting Liu <b28495@freescale.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>