x86/doc/Makefile: DBG=1 to preprocess external asm

The macro hell sometimes make it difficult to trace the source of
an error, so it is easier to analyze the preprocessed output.

This patch makes this automatical by specifying DBG=1 on the
command line: a file ffmpeg/dir/file.asm gets preprocessed to
builddir/dir/file.dbg.asm, which is then compiled.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>