makedef: Pass EXTERN_PREFIX from configure to makedef

This avoids having to use either "dumpbin -headers" to find out
the current architecture, or pass $ARCH from configure to deduce it.

When configuring with --disable-asm, ARCH is equal to "c", which doesn't
give any indication of what symbol prefix is to be used.

Signed-off-by: Martin Storsjö <martin@martin.st>