nios2: allow STANDALONE_LOAD_ADDR overriding

This patch allows users to override default STANDALONE_LOAD_ADDR.
The gcclibdir path was duplicated in the standalone Makefile and
can be removed.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Scott McNutt <smcnutt@psyent.com>