rts: Ensure that pinned allocations respect block size

Previously, it was possible for pinned, aligned allocation requests to
allocate beyond the end of the pinned accumulator block. Specifically,
we failed to account for the padding needed to achieve the requested
alignment in the "large object" check. With large alignment requests,
this can result in the allocator using the capability's pinned object
accumulator block to service a request which is larger than
`PINNED_EMPTY_SIZE`.

To fix this we reorganize `allocatePinned` to consistently account for
the alignment padding in all large object checks. This is a bit subtle
as we must handle the case of a small allocation request filling the
accumulator block, as well as large requests.

Fixes #23400.

(cherry picked from commit fd8c5769)