[Patch] Fix anonymous code atoms problem on Darwin.

[IainS]

Hi,

This problem became apparent during the resolution of PR46904/46916  
(we have had to switch off freorder-and-partition to avoid regression).

--

Darwin's toolchain will not allow us to have code fragments that are  
anonymous (i.e. isolated by section and preceded only by a local label).

The rule [as quoted by Mike] is:
All sections have zero or more atoms.  An atom is 1 or more bytes with  
a label (non-L) on the front of it.

function reordering or partitioning breaks this, manifesting in linker  
warnings and non-functional debug.

The following patch creates and calls a target hook after mid-function  
section switches that gives the target an opportunity to do whatever  
is necessary to avoid orphan code fragments.  The darwin  
implementation of the hook post-fixes the function name with _$hot$ or  
_$cold$ as appropriate.

OK for trunk?
Iain

gcc:

	* target.def (function_switched_text_sections): New Hook.
	* doc/tm.texi Regenerated.
	* doc/tm.texi.in (TARGET_ASM_FUNCTION_SWITCHED_TEXT_SECTIONS)
	New.
	* final.c (default_function_switched_text_sections): New.
	(final_scan_insn): Call function_switched_text_sections when a mid- 
function section
	change occurs.
	* output.h (default_function_switched_text_sections): Declare.
	* config/darwin-protos.h (darwin_function_switched_text_sections) :  
Likewise.
	* config/darwin.c (darwin_function_switched_text_sections): New.
	* config/darwin.h (TARGET_ASM_FUNCTION_SWITCHED_TEXT_SECTIONS)
	New.

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