Objective-C, Darwin : Revise the runtime selection options.

Objective-C, Darwin : Revise the runtime selection options.

For the NeXT/Darwin runtime as installed on macOS, we now
need to deal with changes in ABI and APIs over the range
of system versions supported.

For the first phase of these changes, we will make the
existing flag_next_runtime carry a value that represents
the target OS version.  It might be necessary to extend the
value to represent the ABI ('fragile', V1/V2) or the linker
capabilities (some changes to the linker-visible and public
symbols are dependent on linker version).

TODO: arrange to set the default NEXT_OBJC_RUNTIME on the
basis of the configured target version.

gcc/ChangeLog:

	* config/darwin.c (darwin_objc2_section): Allow for
	values > 1 to represent the next runtime.
	(darwin_objc1_section): Likewise.
	* config/darwin.h (NEXT_OBJC_RUNTIME): Set the default
	next runtime value to be 10.5.8.