[Darwin] Fix PR71767 - adjust the sections used where necessary.

[Darwin] Fix PR71767 - adjust the sections used where necessary.

 (much) Older Darwin linkers needed separate sections marked "coalesce" to
 allow for weak symbol coalescing.  This has not been needed for some time
 and is now deprecated, newer assemblers warn if the old coalesced sections
 are used.

gcc/

2016-11-27  Iain Sandoe  <iain@codesourcery.com>

	PR target/71767
	* config/darwin-sections.def (picbase_thunk_section): New.
	* config/darwin.c (darwin_init_sections): Set up picbase thunk
	section. (darwin_rodata_section, darwin_objc2_section,
	machopic_select_section, darwin_asm_declare_constant_name,
	darwin_emit_weak_or_comdat, darwin_function_section): Don’t use
	coalesced with newer linkers.
	(darwin_override_options): Decide on usage of coalesed sections
	on the basis of the target linker version.
	* config/darwin.h (MIN_LD64_NO_COAL_SECTS): New.
	* config/darwin.opt  (mtarget-linker): New.
	* config/i386/i386.c (ix86_code_end): Do not force the thunks into
	a coalesced section, instead use a thunks section.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@242895 138bc75d-0d04-0410-961f-82ee72b054a4