#NASM x86_64 - <s>349</s> 283 bytes
This should be run 64 bit linux systems

built using:

`nasm loading_golfed.asm -felf64 && ld loading_golfed.o`

	%use altreg
	global _start
	section .data
	o:db"Loading...  "
	s:db"|/-\\"
	b:db`\bx`
	q:dq 0,250000000
	_start:mov r0,1
	mov r7,1
	mov r6,o
	mov r2,12
	syscall
	mov r2,2
	l:mov r7,1
	mov al,[s+r8]
	mov [b+1],al
	mov r0,1
	mov r6,b
	syscall
	mov r0,35
	lea r7,[q]
	mov r6,0
	syscall
	inc r8
	and r8,3
	jmp l

animation: 

saved 65 bytes - thanks user254948

  [![enter image description here][2]][2]

  [2]: https://i.sstatic.net/GErli.gif