[ELF] Simplify getSectionRank

Follow-up to a previous simplification
2473b1af.

The xor difference between a SHT_NOTE and a read-only SHT_PROGBITS
(previously >=NOT_SPECIAL) should be smaller than RF_EXEC. Otherwise,
for the following section layout, `findOrphanPos` would place .text
before note.

```
// simplified from linkerscript/custom-section-type.s
non orphans:
progbits 0x8060c00 NOT_SPECIAL
note     0x8040003

orphan:
.text    0x8061000 NOT_SPECIAL
```