Objective-C, NeXT, v2: Correct a regression in code-gen.

Objective-C, NeXT, v2: Correct a regression in code-gen.

There have been several changes in the ABI of Objective-C which
depend on the OS version targetted.  In this case Protocols and
LabelProtocols should be made weak/hidden/extern from macOS 10.7
however there was a mistake in the code causing this to occur
from macOS 10.6.  Fixed thus.

gcc/objc/ChangeLog:

	* objc-next-runtime-abi-02.c (WEAK_PROTOCOLS_AFTER): New.
	(next_runtime_abi_02_protocol_decl): Use WEAK_PROTOCOLS_AFTER
	to determine this ABI change.
	(build_v2_protocol_list_address_table): Likewise.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
(cherry picked from commit 9b5c0be59d0f94df0517820f00b4520b5abddd8c)