depends: Override host compilers for FreeBSD and OpenBSD

When building depends on FreeBSD/OpenBSD `aarch64`, the host compilers
default to `default_host_{CC,CXX}`, which resolves to `gcc`/`g++`. This
is incorrect on these systems, where Clang is the default system
compiler.

depends/builders/freebsd.mk

@@ -3,3 +3,7 @@ build_freebsd_CXX=clang++
 
 build_freebsd_SHA256SUM = sha256sum
 build_freebsd_DOWNLOAD = curl --location --fail --connect-timeout $(DOWNLOAD_CONNECT_TIMEOUT) --retry $(DOWNLOAD_RETRIES) -o
+
+# freebsd host on freebsd builder: override freebsd host preferences.
+freebsd_CC = clang
+freebsd_CXX = clang++