cmd/cgo: run more gcc invocations concurrently

[matloob]

Currently the cgo command has an initial concurrent phase where it processes each file, and then a sequential phase which processes each file and runs gcc for the errors and debug info. It's pretty difficult to do most processing in parallel, but we can kick off the gcc runs before we start processing and that should help.

[gopherbot]

Change https://go.dev/cl/699019 mentions this issue: cmd/cgo: run recordTypes for each of the debugs at the end of Translate

[gopherbot]

Change https://go.dev/cl/699020 mentions this issue: cmd/cgo: run gcc to get errors and debug info in parallel

[gopherbot]

Change https://go.dev/cl/699015 mentions this issue: cmd/cgo: move typedefs and typedefList out of Package

[gopherbot]

Change https://go.dev/cl/699018 mentions this issue: cmd/cgo: split loadDWARF into two parts