The reason that I did it this way is that the use of LP64 causes issues for CHERI (we use 64-bit virtual addresses and integer registers, so generally do want the LP64 branch, but pointers are 128-bits so LP64 is not defined).
But actually, the SIZEOF_POINTER checks also don't work for CHERI. In the CheriBSD patch I have an additional ifdef first so it does matter what the other branches do.

Since your suggestion also works for all currently supported architectures, and I can make the change if you prefer.

Yes I prefer standard ABI designators. We have around 450 checks with them, not counting contrib.

Well, a lot of those are wrong for CHERI and should become more precise in what their requirements are; most of the time it's really __SIZEOF_LONG__ (or, even more precisely, "how big is my virtual address space", but the two are equivalent if you don't care about Window's awful LLP64). However, __ILP32__ and __LP64__ are fine to be checking here, as pure-capability CHERI C is a different ABI with a different environment variable prefix.

good catch, it was required in the initial version for CheriBSD, but is no longer needed.