Unbreak the build on FreeBSD/i386, where the default target arch is i486.

The recent version of ghc-prim assumes a more modern processor as it
exploits built-in atomic operations, and some of them are not yet
present on i486.  Hence the -march flag is explicitly set to i686 for the
C compiler -- just to be in sync with the default of other distributions.

aclocal.m4

@@ -529,6 +529,9 @@ AC_DEFUN([FPTOOLS_SET_C_LD_FLAGS],
     i386-unknown-mingw32)
         $2="$$2 -march=i686"
         ;;
+    i386-portbld-freebsd*)
+        $2="$$2 -march=i686"
+        ;;
     i386-apple-darwin)
         $2="$$2 -m32"
         $3="$$3 -m32"