include: arch: rx: Change data symbol name

The RX linker is using the name "data" for .data region start
which caused the application cannot use "data" for variable
naming globally.
This commit change the name to "__data_region_start" as expected
in the linker_defs.h file

Signed-off-by: Duy Nguyen <duy.nguyen.xa@renesas.com>