// void fx_call_on_stack(void *stack_top, void (*fn)(void *), void *arg)
// Calls fn(arg) with sp moved to stack_top on the current thread, then restores sp.
.text
.globl _fx_call_on_stack
.p2align 2
_fx_call_on_stack:
    stp x29, x30, [sp, #-16]!
    mov x29, sp
    mov x9, sp
    and x0, x0, #~15
    mov sp, x0
    stp x9, xzr, [sp, #-16]!
    mov x0, x2
    blr x1
    ldp x9, xzr, [sp], #16
    mov sp, x9
    ldp x29, x30, [sp], #16
    ret