rtld: Annotate .rtld_start on aarch64

1. Add a stop indicator to rtld_start to satisfy unwinders on aarch64:
The right unwinding stop indicator should be CFI-undefined PC.
https://dwarfstd.org/doc/Dwarf3.pdf - page 118:
If a Return Address register is defined in the virtual unwind table,
and its rule is undefined (for example, by DW_CFA_undefined), then
there is no return address and no call address, and the virtual
unwind of stack activations is complete.

2. Add a proper annotations for CFA.

That is allows gdb and libunwind successfully stop when unwinding stack
from global constructors and destructors.

Reviewed by:
Differential Revision:	https://reviews.freebsd.org/D40844

libexec/rtld-elf/aarch64/rtld_start.S

@@ -31,15 +31,18 @@
 __FBSDID("$FreeBSD$");
 
 ENTRY(.rtld_start)
+	.cfi_undefined	x30
 	mov	x19, x0		/* Put ps_strings in a callee-saved register */
 
 	sub	sp, sp, #16	/* Make room for obj_main & exit proc */
+	.cfi_adjust_cfa_offset	16
 
 	mov	x1, sp		/* exit_proc */
 	add	x2, x1, #8	/* obj_main */
 	bl	_rtld		/* Call the loader */
 	mov	x8, x0		/* Backup the entry point */
 	ldp	x2, x1, [sp], #16 /* Load cleanup, obj_main */
+	.cfi_adjust_cfa_offset	0
 
 	mov	x0, x19		/* Restore ps_strings */
 	br	x8		/* Jump to the entry point */