arm64: vdso: Fix compilation with clang older than 8

clang versions older than 8 do not support -mcmodel=tiny.

Add a check to the vDSO Makefile for arm64 to remove the flag when
these versions of the compiler are detected.

Reported-by: Qian Cai <cai@lca.pw>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Qian Cai <cai@lca.pw>
Cc: linux-arch@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mips@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: catalin.marinas@arm.com
Cc: will.deacon@arm.com
Cc: arnd@arndb.de
Cc: linux@armlinux.org.uk
Cc: ralf@linux-mips.org
Cc: paul.burton@mips.com
Cc: daniel.lezcano@linaro.org
Cc: salyzyn@android.com
Cc: pcc@google.com
Cc: shuah@kernel.org
Cc: 0x7f454c46@gmail.com
Cc: linux@rasmusvillemoes.dk
Cc: huw@codeweavers.com
Cc: sthotton@marvell.com
Cc: andre.przywara@arm.com
Cc: luto@kernel.org
Link: https://lkml.kernel.org/r/20190626113632.9295-1-vincenzo.frascino@arm.com

arch/arm64/kernel/vdso/Makefile

@@ -38,6 +38,13 @@ else
 CFLAGS_vgettimeofday.o = -O2 -mcmodel=tiny -include $(c-gettimeofday-y)
 endif
 
+# Clang versions less than 8 do not support -mcmodel=tiny
+ifeq ($(CONFIG_CC_IS_CLANG), y)
+  ifeq ($(shell test $(CONFIG_CLANG_VERSION) -lt 80000; echo $$?),0)
+    CFLAGS_REMOVE_vgettimeofday.o += -mcmodel=tiny
+  endif
+endif
+
 # Disable gcov profiling for VDSO code
 GCOV_PROFILE := n