aarch64: Tweak handling of function literal pools

aarch64: Tweak handling of function literal pools

We use AArch32-style text literal pools if:

- -mpc-relative-literal-loads is passed explicitly or
- the code model is “large”

In other cases we put constant pool entries in the data section.
The “tiny” code model then uses PC-relative addressing to access
these data section entries, whereas the “small” model uses a
hi/lo split.

However, we must never put a “real” capability into a text literal pool,
since that would require the text section to be relocated at runtime.
The decision therefore needs to become constant-specific.