Make cpu_relax() nop instructions in one asm block
Make cpu_relax() nop instructions in one asm block

With SKIBOOT_GCOV=1 and multiple asm blocks for nop instruction, we end
up with this:
   1a5f8:       60 00 00 00     nop
   1a5fc:       60 00 00 00     nop
   1a600:       60 00 00 00     nop
   1a604:       60 00 00 00     nop
   1a608:       3d 42 00 0e     addis   r10,r2,14
   1a60c:       e9 2a 20 f8     ld      r9,8440(r10)
   1a610:       39 29 00 01     addi    r9,r9,1
   1a614:       f9 2a 20 f8     std     r9,8440(r10)
   1a618:       60 00 00 00     nop
   1a61c:       60 00 00 00     nop
   1a620:       60 00 00 00     nop
   1a624:       60 00 00 00     nop

Which is not the desired code output for relaxing the cpu.

By batching up the instructions into one asm block, we get the
desired effect: just one set of mcount update

mikey/skiboot,shenki/skiboot,shenki/skiboot,mikey/skiboot,legoater/skiboot,csmart/skiboot,open-power/skiboot,ddstreet/skiboot,open-power/skiboot,ddstreet/skiboot,open-power/skiboot,qemu/skiboot,csmart/skiboot,qemu/skiboot,shenki/skiboot,stewart-ibm/skiboot,apopple/skiboot,qemu/skiboot,mikey/skiboot,ddstreet/skiboot,qemu/skiboot,legoater/skiboot,shenki/skiboot,qemu/skiboot,open-power/skiboot,legoater/skiboot,legoater/skiboot,csmart/skiboot,apopple/skiboot,legoater/skiboot,open-power/skiboot,shenki/skiboot,stewart-ibm/skiboot,apopple/skiboot,stewart-ibm/skiboot