rts: Allow ExecPage to allocate anywhere in address space

Currently the ExecPage facility has two users:

 * GHCi, for constructing info tables, and
 * the adjustor allocation path

Despite neither of these have any spatial locality constraints ExecPage
was using the linker's `mmapAnonForLinker`, which tries hard to ensure
that mappings end up nearby the executable image. This makes adjustor
allocation needlessly subject to fragmentation concerns.

We now instead return less constrained mappings, improving the
robustness of the mechanism.

Addresses #25503.