This fixes the sorting of to copy symbols in lto_output introduced
with GCC 10 to not depend on the actual values of the randomness
we append to LTO section names but instead on the order they appear
in the unsorted array.

This fixed observed debug info differences due to tree merging
prevailing different early debug pointers.

2020-07-06  Richard Biener  <rguenther@suse.de>

	* lto-streamer-out.c (cmp_symbol_files): Use the computed
	order map to sort symbols from the same sub-file together.
	(lto_output): Compute a map of sub-file to an order number
	it appears in the symbol output array.

(cherry picked from commit c50502ca4ebb4d4bd2df9edf1eb5c53aacfa0a1e)