powerpc/64: Fix build failure with allyesconfig in book3s_64_entry.S

Using conditional branches between two files is hasardous,
they may get linked too far from each other.

  arch/powerpc/kvm/book3s_64_entry.o:(.text+0x3ec): relocation truncated
  to fit: R_PPC64_REL14 (stub) against symbol `system_reset_common'
  defined in .text section in arch/powerpc/kernel/head_64.o

Reorganise the code to use non conditional branches.

Fixes: 89d35b23

 ("KVM: PPC: Book3S HV P9: Implement the rest of the P9 path in C")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Avoid odd-looking bne ., use named local labels]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/89cf27bf43ee07a0b2879b9e8e2f5cd6386a3645.1648366338.git.christophe.leroy@csgroup.eu