Higher zstd compression levels for debug info

[ishitatsuyuki]

facebook/zstd#2832 (comment) mentions that:

Starting v1.5.6, zstd at high compression levels features a better compression ratio on 32-bit structures. The .debug_str_offsets section is a pretty good example of this.

Note that compression at level 19 is very slow, and the mentioned optimization only applies to sections where there are lots of 32-bit integers. It will likely not yield benefits for ULEB128-dominated sections. We should test ratio for each section and build a list of sections where zstd-19 is beneficial.

I propose that we introduce new options for --compress-debug-sections:

• zstd-3 - Level 3, would be equivalent to current behavior of zstd
• zstd- Level 19 for 32-bit integer sections, level 3 for the rest
• zstd-19 - Level 19 for everything, in case one is seeking the best ratio

[rui314]

It feels a bit odd to me that we need to manually optimize compression levels. Ideally, zstd should (and I believe could) detect that the data being compressed contains lots of 32-bit integers and compress accordingly. Did you know why they can't do that?