Fix `ARCH_FLAG` when cross-compiling on macOS

configure.ac

@@ -461,7 +461,7 @@ AC_SUBST(CC_VERSION_MESSAGE, $cc_version_message)
 : ${DLDFLAGS="$LDFLAGS"}
 
 RUBY_UNIVERSAL_ARCH
-AS_IF([test "$target_cpu" != "$host_cpu" -a "$GCC" = yes -a "$cross_compiling" = no -a "${universal_binary:-no}" = no], [
+AS_IF([test "$target_cpu" != "$host_cpu" -a "$GCC" = yes -a "${universal_binary:-no}" = no], [
     RUBY_DEFAULT_ARCH("$target_cpu")
 ])
 host_os=$target_os

tool/m4/ruby_default_arch.m4

@@ -9,6 +9,10 @@ AS_CASE([$1:"$host_cpu"],
     [[i[3-6]86]:x86_64], [ARCH_FLAG=-m32],
     [ppc64:ppc*],        [ARCH_FLAG=-m64],
     [ppc*:ppc64],        [ARCH_FLAG=-m32],
+    AS_CASE([$build_os],
+        [darwin*], [ARCH_FLAG="-arch "$1],
+        [ARCH_FLAG=-march=$1]
+    )
 )
 AC_MSG_RESULT([$ARCH_FLAG])
 ])dnl