rtld: Annotate .rtld_start on i386

Add a stop indicator to rtld_start to satisfy unwinders:
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.

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

Reviewed by:		kib
Differential Revision:	https://reviews.freebsd.org/D40949

libexec/rtld-elf/i386/rtld_start.S

@@ -30,8 +30,15 @@
 	.globl	.rtld_start
 	.type	.rtld_start,@function
 .rtld_start:
+	.cfi_startproc
+	.cfi_undefined %eip
 	xorl	%ebp,%ebp		# Clear frame pointer for good form
 	movl	%esp,%esi		# Save initial stack pointer
+	pushl	%ebp
+	.cfi_def_cfa_offset 4
+	movl	%esp,%ebp
+	.cfi_offset %ebp,-4
+	.cfi_def_cfa_register %ebp
 	andl	$0xfffffff0,%esp	# Align stack pointer
 	subl	$16,%esp		# A place to store exit procedure addr
 	movl	%esp,%ebx		# save address of exit proc
@@ -53,6 +60,7 @@
 .globl .rtld_goto_main
 .rtld_goto_main:	# This symbol exists just to make debugging easier.
 	jmp	*%eax			# Enter main program
+	.cfi_endproc
 
 
 /*