cmd/compile/internal/pgo: add hint to missing start_line error
Profiles only began adding Function.start_line in 1.20. If it is
missing, add a hint to the error message that they may need to profile a
build of the application built with a newer version of the toolchain.
Technically profiles are not required to come from Go itself (e.g., they
could be converted from perf), but in practice they most likely are.
Fixes #57674.
Change-Id: I87eca126d3fed0cff94bbb8dd748bd4652f88b12
Reviewed-on: https://go-review.googlesource.com/c/go/+/461195
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>