powerpc/perf: Fix 32bit compile

The "read_bhrb" global symbol is only called under CONFIG_PPC64 of
arch/powerpc/perf/core-book3s.c but it is compiled for both 32 and 64 bit
anyway (and LLVM fails to link this on 32bit).

This fixes it by moving bhrb.o to obj64 targets.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220421025756.571995-1-aik@ozlabs.ru