Fifth argument of la_pltenter() is not constant.

Fifth argument of la_pltenter() is not constant.

The original runtime linker auditing interface described
by Solaris allows the 5th argument of la_pltenter() to be
modified. This patch cleans up the ldsodefs.h definitions
such that the 5th argument is not constant.

At one point the 5th argument *was* constant but this was
changed with commit 2413fdba7a02ba8916f75d17199a6e9133a8f7b0.
This patch updates alpha, ia64, mips, sh and sparc with similar
changes.