arm64: Implement __lshrti3 library function

Commit fb872273 ("arm64: support __int128 on gcc 5+") added support
for the __int128 data type, but this breaks the build in some configurations
where GCC ends up emitting calls to the __lshrti3 helper in libgcc, which
results in a link error:

  kernel/sched/fair.o: In function `__calc_delta':
  fair.c:(.text+0xca0): undefined reference to `__lshrti3'
  kernel/time/timekeeping.o: In function `timekeeping_resume':
  timekeeping.c:(.text+0x3f60): undefined reference to `__lshrti3'
  make: *** [vmlinux] Error 1

Fix the build by providing an implementation of __lshrti3, like we do
already for __ashlti3 and __ashrti3.
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/lib/tishift.S

@@ -57,3 +57,24 @@ ENTRY(__ashrti3)
 	mov	x1, x2
 	ret
 ENDPROC(__ashrti3)
+
+ENTRY(__lshrti3)
+	cbz	x2, 1f
+	mov	x3, #64
+	sub	x3, x3, x2
+	cmp	x3, #0
+	b.le	2f
+	lsr	x0, x0, x2
+	lsl	x3, x1, x3
+	lsr	x2, x1, x2
+	orr	x0, x0, x3
+	mov	x1, x2
+1:
+	ret
+2:
+	neg	w0, w3
+	mov	x2, #0
+	lsr	x0, x1, x0
+	mov	x1, x2
+	ret
+ENDPROC(__lshrti3)