[ping] move and adjust PROBE_STACK_*_REG on aarch64

[Olivier Hainque]

Hello,

After
  https://gcc.gnu.org/pipermail/gcc-patches/2020-January/537843.html
and
  https://gcc.gnu.org/legacy-ml/gcc-patches/2019-12/msg01398.html

Re-proposing this patch after re-testing with a recent
mainline on on aarch64-linux (bootstrap and regression test
with --enable-languages=all), and more than a year of in-house
use in production for a few aarch64 ports on a gcc-9 base.

The change moves the definitions of PROBE_STACK_FIRST_REG
and PROBE_STACK_SECOND_REG to a more appropriate place for such
items (here, in aarch64.md as suggested by Richard), and adjusts
their value from r9/r10 to r10/r11 to free r9 for a possibly
more general purpose (e.g. as a static chain at least on targets
which have a private use of r18, such as Windows or Vxworks).

OK to commit?

Thanks in advance,

With Kind Regards,

Olivier

2020-11-07  Olivier Hainque  <hainque@adacore.com>

	* config/aarch64/aarch64.md: Define PROBE_STACK_FIRST_REGNUM
	and PROBE_STACK_SECOND_REGNUM constants, designating r10/r11.
	Replacements for the PROBE_STACK_FIRST/SECOND_REG constants in
	aarch64.c.
	* config/aarch64/aarch64.c (PROBE_STACK_FIRST_REG): Remove.
	(PROBE_STACK_SECOND_REG): Remove.
	(aarch64_emit_probe_stack_range): Adjust to the _REG -> _REGNUM
	suffix update for PROBE_STACK register numbers.

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