[gdb/symtab] Add sect_off map to partial_symtab

[gdb/symtab] Add sect_off map to partial_symtab

Initially, we added a sect_off field to partial_symbol, but that turned out
to make the bcache aspect of partial symbols ineffective, changing the
"percentage of duplicates" for "partial symbol cache" as reported by "maint
print statistics" from 95% to 0%.

This showed up both as a slow down and more memory usage.

At least parts of the slow down could possibly be addressed by removing the
bcache aspect entirely (given that there are no duplicates), but that wouldn't
address the increase in memory usage.

Instead, implement the sect_off field as a side table.