ac/llvm: export ac_init_llvm_once in targets
If a program like mpv uses both radeon_dri.so (because --vo=gpu) and radeonsi_drv_video.so (because --hwdec=vaapi) then LLVM will be inialized twice.
The commit exports the ac_init_llvm_once so there's only one instance of the function and once_flag variable.
See also 18b12bf5 ("targets: export radeon winsys_create functions to silence LLVM warning") which implemented this workaround initially.
Closes: #1377 (closed)