util: Fix incorrect use of m5_loadsymbol

util: Fix incorrect use of m5_loadsymbol

It seems like the m5 utility incorrectly called m5_loadsymbol instead
of m5_addsymbol. Judging by the signature of the loadsymbol command,
the expected behavior is to add a new symbol to gem5's symbol
table. This is behavior is implemented by m5_addsymbol.

Change-Id: I83b61c48d6f8d7b1e8b57d884dfca00481c83c3a
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2961
Reviewed-by: Gabe Black <gabeblack@google.com>