Downstream issue: arm#482

With this new A320 in-order core, we follow adding the
FeatureUseFixedOverScalableIfEqualCost feature to A510 and A520
(#132246), which reaps the same code generation benefits of preferring fixed
over scalable when the cost is equal.

So when we have:
```
void foo(float* a, float* b, float* dst, unsigned n) {
    for (unsigned i = 0; i < n; ++i)
        dst[i] = a[i] + b[i];
}
```

When compiling without the feature enabled, we get:
```
...
    ld1b    { z0.b }, p0/z, [x0, x10]
    ld1b    { z2.b }, p0/z, [x1, x10]
    add     x12, x0, x10
    ldr     z1, [x12, arm#1, mul vl]
    add     x12, x1, x10
    ldr     z3, [x12, arm#1, mul vl]
    fadd    z0.s, z2.s, z0.s
    add     x12, x2, x10
    fadd    z1.s, z3.s, z1.s
    dech    x11
    st1b    { z0.b }, p0, [x2, x10]
    incb    x10, all, mul arm#2
    str     z1, [x12, arm#1, mul vl]
...
```

When compiling with, we get:
```
...
  	ldp	    q0, q1, [x12, #-16]
	ldp	    q2, q3, [x11, #-16]
	subs	x13, x13, arm#8
	fadd	v0.4s, v2.4s, v0.4s
	fadd	v1.4s, v3.4s, v1.4s
	add	    x11, x11, arm#32
	add	    x12, x12, arm#32
	stp	    q0, q1, [x10, #-16]
	add	    x10, x10, arm#32

...
```

This patch also moves FeatureUseFixedOverScalableIfEqualCost for A510 and A520
from the CPU features to the tune features.