PA: Set up freelist offsets

This CL

*   Introduces `BUILDFLAG(USE_FREELIST_POOL_OFFSETS)`, defaulting
    false, which will gate the alternative implementation of
    PartitionAlloc's freelists.

*   Trivially reorganizes the code in `partition_freelist_entry.h`,
    separating the existing impl into its own header and introducing
    a layer of indirection that allows existing PA code to continue
    simply including the current header.

Bug: 1461983
Change-Id: Ibc30709443729def9e38f715f9f4cf2773c5aa39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4686657


Reviewed-by: Takashi Sakamoto <tasak@google.com>
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1171612}