[XRay] Make xray_fn_idx entries PC-relative

As mentioned by commit c5d38924 (Apr 2020),
PC-relative entries avoid dynamic relocations and can therefore make the
section read-only.

This is similar to D78082 and D78590. We cannot commit to support
compiler/runtime built at different versions, so just don't play with versions.

For Mach-O support (incomplete yet), we use non-temporary `lxray_fn_idx[0-9]+`
symbols. Label differences are represented as a pair of UNSIGNED and SUBTRACTOR
relocations. The SUBTRACTOR external relocation requires r_extern==1 (needs to
reference a symbol table entry) which can be satisfied by `lxray_fn_idx[0-9]+`.
A `lxray_fn_idx[0-9]+` symbol also serves as the atom for this dead-strippable
section (follow-up to commit b9a134aa).

Differential Revision: https://reviews.llvm.org/D152661