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

arch/powerpc/perf/Makefile

@@ -3,11 +3,11 @@
 obj-y				+= callchain.o callchain_$(BITS).o perf_regs.o
 obj-$(CONFIG_COMPAT)		+= callchain_32.o
 
-obj-$(CONFIG_PPC_PERF_CTRS)	+= core-book3s.o bhrb.o
+obj-$(CONFIG_PPC_PERF_CTRS)	+= core-book3s.o
 obj64-$(CONFIG_PPC_PERF_CTRS)	+= ppc970-pmu.o power5-pmu.o \
 				   power5+-pmu.o power6-pmu.o power7-pmu.o \
 				   isa207-common.o power8-pmu.o power9-pmu.o \
-				   generic-compat-pmu.o power10-pmu.o
+				   generic-compat-pmu.o power10-pmu.o bhrb.o
 obj32-$(CONFIG_PPC_PERF_CTRS)	+= mpc7450-pmu.o
 
 obj-$(CONFIG_PPC_POWERNV)	+= imc-pmu.o