arm64: vdso: fix flip/flop vdso build bug

Running "make" on an already compiled kernel tree will rebuild the kernel
even without any modifications:

$ make ARCH=arm64 CROSS_COMPILE=/usr/bin/aarch64-unknown-linux-gnu-
arch/arm64/Makefile:58: CROSS_COMPILE_COMPAT not defined or empty, the compat vDSO will not be built
  CALL    scripts/checksyscalls.sh
  CALL    scripts/atomic/check-atomics.sh
  VDSOCHK arch/arm64/kernel/vdso/vdso.so.dbg
  VDSOSYM include/generated/vdso-offsets.h
  CHK     include/generated/compile.h
  CC      arch/arm64/kernel/signal.o
  CC      arch/arm64/kernel/vdso.o
  CC      arch/arm64/kernel/signal32.o
  LD      arch/arm64/kernel/vdso/vdso.so.dbg
  OBJCOPY arch/arm64/kernel/vdso/vdso.so
  AS      arch/arm64/kernel/vdso/vdso.o
  AR      arch/arm64/kernel/vdso/built-in.a
  AR      arch/arm64/kernel/built-in.a
  GEN     .version
  CHK     include/generated/compile.h
  UPD     include/generated/compile.h
  CC      init/version.o
  AR      init/built-in.a
  LD      vmlinux.o

This is the same bug fixed in commit 92a47286 ("x86/boot: Fix
if_changed build flip/flop bug"). We cannot use two "if_changed" in one
target. Fix this build bug by merging two commands into one function.

Fixes: a7f71a2c ("arm64: compat: Add vDSO")
Fixes: 28b1a824 ("arm64: vdso: Substitute gettimeofday() with C implementation")
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Tested-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Co-developed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
[will: merged in compat fix from Vincenzo and made rule names consistent]
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/vdso/Makefile

@@ -57,8 +57,7 @@ $(obj)/vdso.o : $(obj)/vdso.so
 
 # Link rule for the .so file, .lds has to be first
 $(obj)/vdso.so.dbg: $(obj)/vdso.lds $(obj-vdso) FORCE
-	$(call if_changed,ld)
-	$(call if_changed,vdso_check)
+	$(call if_changed,vdsold_and_vdso_check)
 
 # Strip rule for the .so file
 $(obj)/%.so: OBJCOPYFLAGS := -S
@@ -77,6 +76,9 @@ include/generated/vdso-offsets.h: $(obj)/vdso.so.dbg FORCE
 quiet_cmd_vdsocc = VDSOCC   $@
       cmd_vdsocc = $(CC) $(a_flags) $(c_flags) -c -o $@ $<
 
+quiet_cmd_vdsold_and_vdso_check = LD      $@
+      cmd_vdsold_and_vdso_check = $(cmd_ld); $(cmd_vdso_check)
+
 # Install commands for the unstripped file
 quiet_cmd_vdso_install = INSTALL $@
       cmd_vdso_install = cp $(obj)/$@.dbg $(MODLIB)/vdso/$@

arch/arm64/kernel/vdso32/Makefile

@@ -144,8 +144,7 @@ $(obj)/vdso.so.dbg: $(obj)/vdso.so.raw $(obj)/$(munge) FORCE
 
 # Link rule for the .so file, .lds has to be first
 $(obj)/vdso.so.raw: $(src)/vdso.lds $(obj-vdso) FORCE
-	$(call if_changed,vdsold)
-	$(call if_changed,vdso_check)
+	$(call if_changed,vdsold_and_vdso_check)
 
 # Compilation rules for the vDSO sources
 $(c-obj-vdso): %.o: %.c FORCE
@@ -156,6 +155,9 @@ $(asm-obj-vdso): %.o: %.S FORCE
 	$(call if_changed_dep,vdsoas)
 
 # Actual build commands
+quiet_cmd_vdsold_and_vdso_check = LD      $@
+      cmd_vdsold_and_vdso_check = $(cmd_vdsold); $(cmd_vdso_check)
+
 quiet_cmd_vdsold = VDSOL   $@
       cmd_vdsold = $(COMPATCC) -Wp,-MD,$(depfile) $(VDSO_LDFLAGS) \
                    -Wl,-T $(filter %.lds,$^) $(filter %.o,$^) -o $@