[llvm] ARM: Fix calling convention for gnu half conversion functions (PR #147951)

[David Green via llvm-commits]

davemgreen wrote:

A lot of the older parts of the Arm backend were written a long time ago without much apparent code review or test coverage. With the huge number of valid architecture and platform options that accumulated over the years it can mean that the test coverage is difficult to trust.

https://github.com/llvm/llvm-project/pull/147951