microblaze: Fix microblaze init vectors

Microblaze vector table stores several vectors (reset, user exception,
interrupt, debug exception and hardware exception).
All these functions can be below address 0x10000. If they are, wrong
vector table is genarated because jump is not setup from two instructions
(imm upper 16bit and brai lower 16bit).
Adding specific offset prevent problem if address is below 0x10000.
For this case only brai instruction is used.

Signed-off-by: Michal Simek <monstr@monstr.eu>