boards: arc: explicitly set link lib for hs boards with mwdt

arcmwdt toolchain has various pre build libraries, we can find it in
path: {METAWARE_HOME}/lib, and it will use av2em lib by default.
it's OK for em boards, but not suitable for hs boards. I have tested
hs38_full library for hs boards, it's OK. let's use it.

Signed-off-by: Watson Zeng <zhiwei@synopsys.com>