vendorcode/amd/agesa: Use macros already defined in stdlib.h

We already have these macros define in 'stdlib.h'. Make good use of them
here to avoid redefinition conflicts of the pre-processor depending on
header inclusion ordering. This has the nice side-effect of syncing up
AGESA families in this particular regard.

Change-Id: Icf911629a4a1a82b01062fe16af4c8f812b05717
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/6199
Tested-by: build bot (Jenkins)
Reviewed-by: Bruce Griffith <Bruce.Griffith@se-eng.com>