arm64: vdso32: make vdso32 install conditional

vdso32 should only be installed if CONFIG_COMPAT_VDSO is enabled,
since it's not even supposed to be compiled otherwise, and arm64
builds without a 32bit crosscompiler will fail.

Fixes: 8d75785a81 ("ARM64: vdso32: Install vdso32 from vdso_install")
Signed-off-by: Frank van der Linden <fllinden@amazon.com>
Cc: stable@vger.kernel.org [5.4+]
Link: https://lore.kernel.org/r/20200827234012.19757-1-fllinden@amazon.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/Makefile

@@ -170,7 +170,8 @@ zinstall install:
 PHONY += vdso_install
 vdso_install:
 	$(Q)$(MAKE) $(build)=arch/arm64/kernel/vdso $@
-	$(Q)$(MAKE) $(build)=arch/arm64/kernel/vdso32 $@
+	$(if $(CONFIG_COMPAT_VDSO), \
+		$(Q)$(MAKE) $(build)=arch/arm64/kernel/vdso32 $@)
 
 # We use MRPROPER_FILES and CLEAN_FILES now
 archclean: