blackfin: set GNU_TARGET_NAME accordingly

Blackfin for FDPIC targets uses *-linux-*, however for FLAT targets it
needs *-uclinux-* in order for gcc to build properly.

[Thomas: use a TARGET_OS variable instead of redefining
GNU_TARGET_NAME completely]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>