arch: arm: Use BL instead of B to jump to _PrepC

The __reset vector is using the 'b' instruction to jump to _PrepC. 'b'
can only jump 2KB on Cortex-M0 platforms so this causes build failures
when _PrepC happens to be too far away from __reset.

To resolve this we use 'bl' instead as it can jump two thousand times
further[0].

This can also be resolved by porting the reset vector to C or by
placing the _PrepC function in a dedicated section that the linker
places next to __reset.

[0] http://infocenter.arm.com/help/topic/com.arm.doc.dui0489e/Cihfddaf.html



Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>