Fix pool (mis)compile on AArch64

[rjsberry]

Fixes #349

Alternatively we could change the probe to something like:

#[no_mangle]
pub unsafe fn probe(x: &usize, y: usize) -> usize {
    let mut ret;
    asm!(
        "       clrex",
        "2:",
        "       ldrex {2}, [{0}]",
        "       strex {tmp}, {1}, [{0}]",
        "       cmp {tmp}, #0",
        "       bne 2b",
        in(reg) x,
        in(reg) y,
        out(reg) ret,
        tmp = out(reg) _,
    );
    ret
}

Without explicitly denying any aarch64* target.

[AdinAck]

Please merge this. Tests cannot be run on Darwin processors without this PR.

[newAM]

Thanks for the fix! Will merge soon after CI is fixed.