arm64: support __int128 with clang

Commit fb8722735f50 ("arm64: support __int128 on gcc 5+") added support
for arm64 __int128 with gcc with a version-conditional, but neglected to
enable this for clang, which in fact appears to support aarch64 __int128.
This commit therefore enables it if the compiler is clang, using the
same type of makefile conditional used elsewhere in the tree.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/Makefile

@@ -56,7 +56,11 @@ KBUILD_AFLAGS	+= $(lseinstr) $(brokengasinst)
 KBUILD_CFLAGS	+= $(call cc-option,-mabi=lp64)
 KBUILD_AFLAGS	+= $(call cc-option,-mabi=lp64)
 
+ifeq ($(cc-name),clang)
+KBUILD_CFLAGS	+= -DCONFIG_ARCH_SUPPORTS_INT128
+else
 KBUILD_CFLAGS	+= $(call cc-ifversion, -ge, 0500, -DCONFIG_ARCH_SUPPORTS_INT128)
+endif
 
 ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
 KBUILD_CPPFLAGS	+= -mbig-endian