aarch64: Fix pac-ret with unusual dwarf in libgcc unwinder [PR104689]

aarch64: Fix pac-ret with unusual dwarf in libgcc unwinder [PR104689]

The RA_SIGN_STATE dwarf pseudo-register is normally only set using the
DW_CFA_AARCH64_negate_ra_state (== DW_CFA_window_save) operation which
toggles the return address signedness state (the default state is 0).
(It may be set by remember/restore_state CFI too, those save/restore
the state of all registers.)

However RA_SIGN_STATE can be set directly via DW_CFA_val_expression too.
GCC does not generate such CFI but some other compilers reportedly do.

Note: the toggle operation must not be mixed with other dwarf register
rule CFI within the same CIE and FDE.

In libgcc we assume REG_UNSAVED means the RA_STATE is set using toggle
operations, otherwise we assume its value is set by other CFI.

libgcc/ChangeLog:

	PR target/104689
	* config/aarch64/aarch64-unwind.h (aarch64_frob_update_context):
	Handle the !REG_UNSAVED case.
	* unwind-dw2.c (execute_cfa_program): Fail toggle if !REG_UNSAVED.

gcc/testsuite/ChangeLog:

	PR target/104689
	* gcc.target/aarch64/pr104689.c: New test.