[llvm] [ARM] Improve comment on the 'J' inline asm modifier. (PR #160712)

[Peter Smith via llvm-commits]

https://github.com/smithp35 approved this pull request.

LGTM

The only reference I can find is the Arm inline assembly cookbook which has a table near to `For ARM processors, GCC 4 provides the following constraints.` in:
http://www.ethernut.de/en/documents/arm-inline-asm.html

It does mention a similar example to yours.

https://github.com/llvm/llvm-project/pull/160712