devel/llvm16: backport fix for generate arm_neon.h

Backport db49231: [clang][BFloat] Avoid redefining bfloat16_t in
arm_neon.h.

This fixes builds of FreeBSD/aarch64. The change was merged
to the llvm16 in src prior to commit.

(cherry picked from commit 4b887326f2a149bbcf9adb2ab4365aeea4116cbc)