objtool: Add mcsafe_handle_tail() to the uaccess safe list

After an objtool improvement, it's reporting that __memcpy_mcsafe() is
calling mcsafe_handle_tail() with AC=1:

  arch/x86/lib/memcpy_64.o: warning: objtool: .fixup+0x13: call to mcsafe_handle_tail() with UACCESS enabled
  arch/x86/lib/memcpy_64.o: warning: objtool:   __memcpy_mcsafe()+0x34: (alt)
  arch/x86/lib/memcpy_64.o: warning: objtool:   __memcpy_mcsafe()+0xb: (branch)
  arch/x86/lib/memcpy_64.o: warning: objtool:   __memcpy_mcsafe()+0x0: <=== (func)

mcsafe_handle_tail() is basically an extension of __memcpy_mcsafe(), so
AC=1 is supposed to be set.  Add mcsafe_handle_tail() to the uaccess
safe list.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/035c38f7eac845281d3c3d36749144982e06e58c.1563413318.git.jpoimboe@redhat.com