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>