Some clang headers are conflicting with FreeBSD headers. There was a previous patch that removed them from package install, but it was reverted on revision r459483 as it was believe that now works properly.
I identified that conflict still exists, during my tests cross compiling from x86_64 to powerpc64 host using llvm-devel package.
This change reintegrates the previous patch and include more files, matching the ones listed on /usr/src/lib/clang/headers/Makefile that are removed from src BASE:
# Headers which possibly conflict with our own versions:
.if defined(INSTALL_CONFLICTING_CLANG_HEADERS)
INCS+= float.h
INCS+= intrin.h
INCS+= inttypes.h
INCS+= iso646.h
INCS+= limits.h
INCS+= stdalign.h
INCS+= stdarg.h
INCS+= stdatomic.h
INCS+= stdbool.h
INCS+= stddef.h
INCS+= stdint.h
INCS+= stdnoreturn.h
INCS+= tgmath.h
INCS+= unwind.h
INCS+= varargs.h
.endif