rs6000: Rework ELFv2 support for -fpatchable-function-entry* [PR99888]

rs6000: Rework ELFv2 support for -fpatchable-function-entry* [PR99888]

As PR99888 and its related show, the current support for
-fpatchable-function-entry on powerpc ELFv2 doesn't work
well with global entry existence.  For example, with one
command line option -fpatchable-function-entry=3,2, it got
below w/o this patch:

  .LPFE1:
	  nop
	  nop
	  .type   foo, @function
  foo:
	  nop
  .LFB0:
	  .cfi_startproc
  .LCF0:
  0:      addis 2,12,.TOC.-.LCF0@ha
	  addi 2,2,.TOC.-.LCF0@l
	  .localentry     foo,.-foo

, the assembly is unexpected since the patched nops have
no effects when being entered from local entry.

This patch is to update the nops patched before and after
local entry, it looks like:

	  .type   foo, @function
  foo:
  .LFB0:
	  .cfi_startproc
  .LCF0:
  0:      addis 2,12,.TOC.-.LCF0@ha
	  addi 2,2,.TOC.-.LCF0@l
	  nop
	  nop
	  .localentry     foo,.-foo
	  nop

	PR target/99888
	PR target/105649

gcc/ChangeLog:

	* doc/invoke.texi (option -fpatchable-function-entry): Adjust the
	documentation for PowerPC ELFv2 ABI dual entry points.
	* config/rs6000/rs6000-internal.h
	(rs6000_print_patchable_function_entry): New function declaration.
	* config/rs6000/rs6000-logue.cc (rs6000_output_function_prologue):
	Support patchable-function-entry by emitting nops before and after
	local entry for the function that needs global entry.
	* config/rs6000/rs6000.cc (rs6000_print_patchable_function_entry): Skip
	the function that needs global entry till global entry has been
	emitted.
	* config/rs6000/rs6000.h (struct machine_function): New bool member
	global_entry_emitted.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr99888-1.c: New test.
	* gcc.target/powerpc/pr99888-2.c: New test.
	* gcc.target/powerpc/pr99888-3.c: New test.
	* gcc.target/powerpc/pr99888-4.c: New test.
	* gcc.target/powerpc/pr99888-5.c: New test.
	* gcc.target/powerpc/pr99888-6.c: New test.
	* c-c++-common/patchable_function_entry-default.c: Adjust for
	powerpc_elfv2 to avoid compilation error.