Rewrite/fix cygwin1.dbg generation

[Brian Dessent]

Pedro Alves wrote:

> The dllfixdbg hunk looks hard to read.  Here's what is looks
> like after patching:

I think that if whatever bugs used to exist in older binutils PE support
that necessitated this hackery are now gone, we can just do away with
dllfixdbg alltogether and just put this:

> ${STRIP} --strip-debug ${DLL} -o stripped-${DLL}
> ${STRIP} --only-keep-debug ${DLL} -o ${DBG}
> ${OBJCOPY} --add-gnu-debuglink=${DBG} stripped-${DLL} ${DLL}
> rm -f stripped-${DLL}

...in the Makefile.

Brian