avr32: Fix out-of-range rcalls in large kernels

Replace handcoded rcall instructions with the call pseudo-instruction.
For kernels too far over 1MB the rcall instruction can't reach and
linking will fail.  We already call the final linker with --relax which
converts call pseudo-instructions to the right things anyway.

This fixes

arch/avr32/kernel/built-in.o: In function `syscall_exit_work':
(.ex.text+0x198): relocation truncated to fit: R_AVR32_22H_PCREL against symbol `schedule' defined in .sched.text section in kernel/built-in.o
arch/avr32/kernel/built-in.o: In function `fault_exit_work':
(.ex.text+0x3b6): relocation truncated to fit: R_AVR32_22H_PCREL against symbol `schedule' defined in .sched.text section in kernel/built-in.o

But I'm still left with

arch/avr32/kernel/built-in.o:(.fixup+0x2): relocation truncated to fit: R_AVR32_22H_PCREL against `.text'+45a
arch/avr32/kernel/built-in.o:(.fixup+0x8): relocation truncated to fit: R_AVR32_22H_PCREL against `.text'+8ea
arch/avr32/kernel/built-in.o:(....