python3: Enable FreeBSD cross build

This only enables x86_64-unknown-freebsd, not any other FreeBSD, since I
do not have any other machines to tests on. If you're reading this, feel
free to try out other arches!

Signed-off-by: Jörg Thalheim <joerg@thalheim.io>

Author: rhelmot

Misc/NEWS.d/next/Build/2024-09-24-08-30-25.gh-issue-124424.UBXNFo.rst

@@ -0,0 +1 @@
+Enable cross-compiling for x86_64-freebsd

configure.ac

@@ -339,6 +339,9 @@ then
 	*-*-wasi)
 	    ac_sys_system=WASI
 	    ;;
+	*-*-freebsd)
+	    ac_sys_system=FreeBSD
+	    ;;
 	*)
 		# for now, limit cross builds to known configurations
 		MACHDEP="unknown"
@@ -777,6 +780,9 @@ if test "$cross_compiling" = yes; then
 	wasm32-*-* | wasm64-*-*)
 		_host_ident=$host_cpu
 		;;
+	x86_64-*-freebsd)
+		_host_cpu=x86_64
+		;;
 	*)
 		# for now, limit cross builds to known configurations
 		MACHDEP="unknown"