Profile's Mapping field is currently populated by reading /proc/self/maps.
On systems where /proc/self/maps is not available, the profile generated
by Go's runtime will not have any Mapping entry. Pprof command then adds
a fake entry and links all Location entries in the profile with the fake
entry to be used during symbolization.
https://github.com/google/pprof/blob/a8644067d5a3c9a6386e7c88fa4a3d9d37877ca3/internal/driver/fetch.go#L437

The fake entry is not enough to suppress the error or warning messages
pprof command produces. We need to tell pprof that Location entries are
symbolized already by Go runtime and pprof does not have to attempt to
perform further symbolization.

In #25743, we made Go runtime mark Mapping entries with HasFunctions=true
when all Location entries from the Mapping entries are successfully
symbolized. This change makes the Go runtime add a fake mapping entry,
otherwise the pprof command tool would add, and set the HasFunctions=true
following the same logic taken when the real mapping information is
available.

Updates #19790.
Fixes #26255. Tested pprof doesn't report the error message any more
for pure Go program.

Change-Id: Ib12b62e15073f5d6c80967e44b3e8709277c11bd
Reviewed-on: https://go-review.googlesource.com/123779
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>