scons: Fix the compiler flag used for partial linking.

scons: Fix the compiler flag used for partial linking.

It seems that g++ can generally handle the -r flag for generating a
relocatable object file, but ld can't always handle the  --relocatable
flag.

Change-Id: I15f32e469590a814131d4e992b392a7ad6c52b83
Reviewed-on: https://gem5-review.googlesource.com/3001
Reviewed-by: Andreas Hansson <andreas.hansson@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>