runtime/coverage: runtime routines to emit coverage data
This patch fleshes out the runtime support for emitting coverage data
at the end of a run of an instrumented binary. Data is emitted in the
form of a pair of files, a meta-out-file and counter-data-outfile,
each written to the dir GOCOVERDIR. The meta-out-file is emitted only
if required; no need to emit again if an existing meta-data file with
the same hash and length is present.
Updates #51430.
Change-Id: I59d20a4b8c05910c933ee29527972f8e401b1685
Reviewed-on: https://go-review.googlesource.com/c/go/+/355451 Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Than McIntosh <thanm@google.com>