go-fuzz: make CoverTab size tunable

[josharian]

In #249, @dvyukov wrote:

Re CoverTab, it always was supposed to be of tunable size. It's just that I did fixed 64K initially as the simplest option and then never had time/need to improve that.

This issue is to track/discuss this.

From previous experience changing CoverTab size manually to a different hard-coded value, here's the places that need updating:

• Generate code that masks down a high-width counter at runtime. (As Dmitry wrote: CoverTab[LOCATION_HASH & GlobalSizeMask]++.) For folks who just want to tweak CoverTab manually, you can also just change func genCounter in go-fuzz-build to generate different width counters.
• const CoverSize in go-fuzz-defs.
• go-fuzz/compare_amd64.s assembly implementations for clearing the covertab. Probably easiest to just pass in log2(CoverSize). And require that CoverSize be a power of two. Look for the BTL instructions.

The other TODO here is to do a bit of performance tuning with higher values of CoverSize. My experience is that cranking up the size of covertab uncovers new performance pain points. I've fixed a few of these, but more remain.

cc @thepudds

[catenacyber]

The 64K value is also hardcoded for libFuzzer mode if I understand correctly https://github.com/dvyukov/go-fuzz/blob/ee722ec171a1c6df2eb8fc2e36923c4ee5780698/go-fuzz-build/main.go line 637 and next ones