GCC: ARM_CM0: Fix L6286E error on Keil MDK (#1131)

GCC: ARM_CM0: Fix L6286E error on Keil MDK (#1131)

Change the .b instruction to .bx with higher range to solve error
reported by MDK descibed bellow.

Fix:
Error: L6286E: Relocation #REL:0 in portasm.o(.text.SVC_Handler) with respect to vPortSVCHandler_C. Value(0x1a04) out of range(-0x800 - 0x7fe) for (R_ARM_THM_JUMP11)

Compiler: Keil MDK ARMClang 6.22.0

https://developer.arm.com/documentation/ka002847/latest/
https://developer.arm.com/documentation/dui0496/m/Linker-Errors-and-Warnings/List-of-the-armlink-error-and-warning-messages

Signed-off-by: Tomas Galbicka <tomas.galbicka@nxp.com>