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>

src/internal/coverage/rtcov/rtcov.go

@@ -23,3 +23,12 @@ type CovMetaBlob struct {
 	CounterMode        uint8 // coverage.CounterMode
 	CounterGranularity uint8 // coverage.CounterGranularity
 }
+
+// CovCounterBlob is a container for encapsulating a counter section
+// (BSS variable) for an instrumented Go module. Here "counters"
+// points to the counter payload and "len" is the number of uint32
+// entries in the section.
+type CovCounterBlob struct {
+	Counters *uint32
+	Len      uint64
+}