Fix minor PG_CACHE_LINE_SIZE bitrot
Submitted patch for B-Tree support
Make B-Tree/CLUSTER sortsupport use abbreviation

This commit is intended to be applied on top of the bitrot-fixing commit
that was attached alongside it.  Then, apply the abbreviated key support
patch, which was posted here:

CAM3SWZT+V3LLRBSCyUJ9jOvuMDpBq6bUpF9woFBfuyWgwuHW3A@mail.gmail.com.

In addition, the B-Tree/CLUSTER sortsupport patch must then be applied:

CAM3SWZTfKZHTUiWDdHg+6tcYuMsdHoC=bMuAiVgMP9AThj42Gw@mail.gmail.com

B-Tree sortsupport with abbreviated keys shows benefits (and costs) that
are similar to the heavily tested MinimalTuple case.  There is
additional overhead when building a B-Tree index as compared to heap
tuple sorts (with no client overhead), but even still the vast majority
of system time is spent actually sorting index tuples.