RFC: Add STB_GNU_SECONDARY

[Joern Rennecke]

Quoting "H.J. Lu" <hjl.tools@gmail.com>:

> Hi,
>
> We have a need to define a secondary symbol as backup in
> case there isn't a primary one.  Here is a proposal for
> STB_GNU_SECONDARY.  Any comments?

If two levels of prevedence (ordinary and weak) are not enough, why will
three levels be so much better?
If you use a signed fractional or even floating-point precedence value,
you have a lot more space to accomodate afterthoughts - above, below,
and in-between in precedence to existing values.

Even better, you could use symbolic tags, and have the linker script
assign precedence values to these tags.