Enable dynamic libraries on OSX amd64

I don't know why it wasn't already enabled, but it seems to work.

mk/config.mk.in

@@ -114,11 +114,12 @@ GhcDynamic=NO
 GhcProfiled=NO
 
 # Do we support shared libs?
-SharedLibsPlatformList = i386-unknown-linux x86_64-unknown-linux \
-	i386-unknown-freebsd x86_64-unknown-freebsd \
-	i386-unknown-openbsd x86_64-unknown-openbsd \
-	i386-unknown-mingw32 \
-	i386-apple-darwin powerpc-apple-darwin
+SharedLibsPlatformList =                        \
+    i386-unknown-linux   x86_64-unknown-linux   \
+    i386-unknown-freebsd x86_64-unknown-freebsd \
+    i386-unknown-openbsd x86_64-unknown-openbsd \
+    i386-unknown-mingw32                        \
+    i386-apple-darwin    x86_64-apple-darwin    powerpc-apple-darwin
 
 ifeq "$(SOLARIS_BROKEN_SHLD)" "NO"
 SharedLibsPlatformList += i386-unknown-solaris2