aarch64: ensure bti c is emitted at function start [PR94697]

aarch64: ensure bti c is emitted at function start [PR94697]

The bti pass currently first emits bti c at function start
if there is no paciasp (which also acts as indirect call
landing pad), then bti j is emitted at jump labels, however
if there is a label right before paciasp then the function
start can end up like

  foo:
  label:
    bti j
    paciasp
    ...

This patch is a minimal fix that just moves the bti c handling
after the bti j handling so we end up with

  foo:
    bti c
  label:
    bti j
    paciasp
    ...

This could be improved by emitting bti jc in this case, or by
detecting that the label is not in fact an indirect jump target
and then this situation would be much less common.

Needs to be backported to gcc-9 branch.

Backported without the testcase because of missing infrastructure
for check-function-bodies.

gcc/ChangeLog:

	Backport from mainline.
	2020-04-23  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	PR target/94697
	* config/aarch64/aarch64-bti-insert.c (rest_of_insert_bti): Swap
	bti c and bti j handling.