RFC: Add --plugin-gcc option to ar/nm

[H.J. Lu]

Hi,

---plugin option for ar/nm is very long.  I am proposing to add
a  --plugin-gcc option.  It can be implemented with

1. Move LTOPLUGINSONAME from gcc to config/plugins.m4.
2. Define LTOPLUGINSONAME for ar/nm.
3. For --plugin-gcc, ar/nm call popen using environment variable GCC if set,
or gcc with -print-prog-name=$LTOPLUGINSONAM to get
plugin name.

Any comments?

-- 
H.J.