devel/llvm[789]: restore arm_bf16.h to ARM backend

arm_bf16.h is generated for both armv7 and aarch64 so move it back to
ARM. We don't support AArch64 without ARM so we can just move it and
not need to worry about duplicate supression (I'd previously fixed the
issue in llvm19 in a more complicated manner so this change removes the
duplicate supression and documents why it's not needed.)

PR: 281179
Reported by: Mark Millard