https://github.com/sharow/libconcurrent/pull/15

--- makefile.orig	2021-09-27 11:43:44 UTC
+++ makefile
@@ -66,6 +66,11 @@ else ifeq ($(LIBCONCURRENT_ARCH),arm64) # MacOS, iOS, 
   LIBCONCURRENT_ARCH_BITS=64
   AS=as
   ASFLAGS=
+ else ifeq ($(SYSTEM), FreeBSD)
+  ARCH=aarch64
+  ARCH_BITS=64
+  AS=as
+  ASFLAGS=
  endif
 else ifeq ($(LIBCONCURRENT_ARCH), aarch64) # Linux
  LIBCONCURRENT_ARCH=aarch64