#NASM x86_64 - 349 283 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

NASM x86_64 - 349 283 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

#NASM x86_64 - 349 283 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

#NASM x86_64 - 349 283 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

#NASM x86_64 - 349 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 r6,s
mov r2,1
syscall
xor r8,r8
mov r6,b
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]
xor r6,r6
syscall
inc r8
cmp r8,4
jl l
xor r8,r8
jmp l

animation:

#NASM x86_64 - 349 283 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