lmbench: fix build with libtirpc

When linking with an external libtirpc, -ltirpc must come after lmbench.a in
the linker command line. Use LDLIBS for that.

Fixes:
http://autobuild.buildroot.net/results/89e/89ee35561d3a49f38e9a747ed78a5846be4764c8/
http://autobuild.buildroot.net/results/409/4097b1c2585db2de517a8741174c02177aabee00/



and more.

Also, remove LMBENCH_LDLIBS assignment. This variable was not used before.

Reviewed-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Tested-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>