i386: Fix wrong codegen for -mrelax-cmpxchg-loop

i386: Fix wrong codegen for -mrelax-cmpxchg-loop

For -mrelax-cmpxchg-loop introduced by PR 103069/r12-5265, it would
produce infinite loop. The correct code should be

.L84:
        movl    (%rdi), %ecx
        movl    %eax, %edx
        orl     %esi, %edx
        cmpl    %eax, %ecx
        jne     .L82
        lock cmpxchgl   %edx, (%rdi)
        jne     .L84
	movl    %r8d, %eax  <<< retval is missing in previous impl
	ret
.L82:
        rep nop
        jmp     .L84

Adjust corresponding expander to fix such issue, and fix runtime test
so the problem would be exposed.

gcc/ChangeLog:

	* config/i386/i386-expand.c (ix86_expand_atomic_fetch_op_loop):
	Adjust generated cfg to avoid infinite loop.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr103069-2.c: Adjust.