[AArch64] PR92424: Fix -fpatchable-function-entry=N,M with BTI

This is a workaround that emits a BTI after the function label if that
is followed by a patch area. We try to remove the BTI that follows the
patch area (this may fail e.g. if the first instruction is a PACIASP).

So before this commit -fpatchable-function-entry=3,1 with bti generates

    .section __patchable_function_entries
    .8byte .LPFE
    .text
  .LPFE:
    nop
  foo:
    nop
    nop
    bti c // or paciasp
    ...

and after this commit

    .section __patchable_function_entries
    .8byte .LPFE
    .text
  .LPFE:
    nop
  foo:
    bti c
    nop
    nop
    // may be paciasp
    ...

and with -fpatchable-function-entry=1 (M=0) the code now is

  foo:
    bti c
    .section __patchable_function_entries
    .8byte .LPFE
    .text
  .LPFE:
    nop
    // may be paciasp
    ...

There is a new bti insn in the middle of the patchable area users need
to be aware of unless M=0 (patch area is after the new bti) or M=N
(patch area is before the label, no new bti). Note: bti is not added to
all functions consistently (it can be turned off per function using a
target attribute or the compiler may detect that the function is never
called indirectly), so if bti is inserted in the middle of a patch area
then user code needs to deal with detecting it.

Tested on aarch64-none-linux-gnu.

gcc/ChangeLog:

	PR target/92424
	* config/aarch64/aarch64.c (aarch64_declare_function_name): Set
	cfun->machine->label_is_assembled.
	(aarch64_print_patchable_function_entry): New.
	(TARGET_ASM_PRINT_PATCHABLE_FUNCTION_ENTRY): Define.
	* config/aarch64/aarch64.h (struct machine_function): New field,
	label_is_assembled.

gcc/testsuite/ChangeLog:

	PR target/92424
	* gcc.target/aarch64/pr92424-2.c: New test.
	* gcc.target/aarch64/pr92424-3.c: New test.