[PATCH] Elf: Adjust compare_link_order for unstable qsort.

[Sandra Loosemore]

In a cross toolchain for nios2-elf target and x86_64-w64-mingw32 host 
using binutils 2.37, we observed a failure that didn't show up on 
x86_64-linux-gnu host:  testcase pr25490-5.s was failing with

C:\path\to\nios2-elf-ld.exe: looping in map_segments
FAIL: __patchable_function_entries section 5

I tracked this down to the SHF_LINK_ORDER sorting added in commit 
b209b5a6b8a4433be961a0f016439f381de65bfc.  Having run into similar 
problems before, I know that the qsort implementation provided by 
Windows host is not a stable sort, and sure enough, that was what was 
going wrong here too.  So I made the comparison function work harder to 
avoid returning 0; it's already falling back to using the idx field 
comparison at an earlier point in the function and I think that is 
exactly what we want to prevent the unstable qsort from flipping the 
order of entries that otherwise compare as equal.

OK for trunk?  Besides testing this in the aforementioned nios2 
cross-toolchain, I also checked that it worked OK on a native 
x86_64-linux-gnu trunk build.

-Sandra
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