[KernelCache] Set segment permissions based on how XNU initially maps them

XNU maps kernel cache segments in with different permissions than the
load commands indicate. For instance, `__DATA_CONST` is initially mapped
as read-write before later being re-mapped as read-only. Treating it as
read-only results in analysis falsely assuming that global variables
cannot change.

To work around this we maintain a mapping from segment name to initial
permissions (i.e., most lax permissions) and favor them over permissions
derived from the segment load command. Section semantics are also
derived from the segment's permissions when the segment is present in
the mapping.

The mapping is based on the initial permissions established by
`arm_vm_prot_init` within the XNU source.

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