Support product partition for non-treblized devices

Non-treblized devices use ld.config.legacy.txt, which does not
support product partition, leading to access denial from/to product partition.
Declare directly /product since search paths are resolved in linker config.

Test: m -j with non-treblized device upgraded to P.
Change-Id: Ic142b807f5dbffdfa5c774b3df8d0903b9626b6a