Objective-C, NeXT: Reorganise meta-data declarations.

Objective-C, NeXT: Reorganise meta-data declarations.

This moves the GTY declaration of the meta-data indentifier
array into the header that enumerates these and provides
shorthand defines for them.  This avoids a problem seen with
a relocatable PCH implementation.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
gcc/objc/ChangeLog:

	* objc-next-metadata-tags.h (objc_rt_trees): Declare here.
	* objc-next-runtime-abi-01.c: Remove from here.
	* objc-next-runtime-abi-02.c: Likewise.
	* objc-runtime-shared-support.c: Reorder headers, provide
	a GTY declaration the definition of objc_rt_trees.