powerpc: add --bss-plt to LDFLAGS

With some versions of gcc (that we know of 4.6.3 and 4.8.2 are affected)
it is necessary to specify --bss-plt to get the final blrl in the
_GOT2_TABLE_. Without this the last symbol does not get it's address
relocated.  For the P2041RDB board this ended up being
NetArpWaitTimerStart which caused the ARP packets to timeout
immediately.

Signed-off-by: Joakim Tjernlund <joakim.tjernlund@transmode.se>
Signed-off-by: Chris Packham <judge.packham@gmail.com>
Acked-by: Joakim Tjernlund <joakim.tjernlund@transmode.se>
Acked-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>