Fix libc-abis handling for add-ons

[Roland McGrath]

> I don't see how that would work in general.  MIPS_PLT had its ABI version 
> before UNIQUE, so needs to come first, but if a ports architecture gets 
> IFUNC support in future then that postdates UNIQUE and so must go after 
> the target-independent definitions.  So there isn't a single order of 
> concatenation for this; ports definitions may need to go both before and 
> after libc definitions.

For the truly general case, we might indeed need to figure out something
slightly more complicated.  But for the present reality, I'm confused as to
how this can be.  Before now, no ld set EI_ABIVERSION.  The first ld that
ever will already supports STB_GNU_UNIQUE.  So how can it possibly be that
MIPS needs to put some other feature before UNIQUE?


Thanks,
Roland