llext: arm: Add R_ARM_ARM_THM_CALL reloc support

Add support for the relocation type R_ARM_ARM_THM_CALL which is
produced for the ARM Thumb BL and BLX (branch immediate)
instructions.

These instructions are used for non-static functions like

void test1(void)
{
}

void main(void)
{
        test1();
}

Without support for this relocation, test1() has to be static.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>