x86:ensure that getcontext will return 0.

The setcontext method did not alway set the return value to 0 after
restoring the desired context. Specially When calling setcontext with
the _UC_IGNSIGM and the _UC_IGNFPU flags the return value would be non
zero.

Change-Id: Iec7f8d6a680950aa53e3c88c86e03f65005e66b2

lib/libc/arch/i386/sys-minix/ucontext.S

@@ -108,6 +108,7 @@ ENTRY(setcontext)
 	mov SI(%edx), %esi		/* Restore ESI */
 	mov BP(%edx), %ebp		/* Restore EBP */
 	mov SP(%edx), %esp		/* Restore stack pointer */
+	xor %eax, %eax			/* Return 0 */
 	jmp *PC(%edx)  			/* Return to RTA */
 
 /* void ctx_start((void *func)(int arg1, ..., argn), arg1, ..., argn,