objtool: Fix STACK_FRAME_NON_STANDARD for cold subfunctions

The recent STACK_FRAME_NON_STANDARD refactoring forgot about .cold
subfunctions.  They must also be ignored.

Fixes the following warning:

  drivers/gpu/drm/vmwgfx/vmwgfx_msg.o: warning: objtool: vmw_recv_msg.cold+0x0: unreachable instruction

Fixes: c84301d7 ("objtool: Ignore entire functions rather than instructions")
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/70a09ec0b0704398b2bbfb3153ce3d7cb8a381be.1743136205.git.jpoimboe@kernel.org