cmd/compile: modify -memprofile flag for multiple profiles in a directory
This permits collection of multiple profiles in a build
(instead of just the last compilation). If a -memprofile
specifies an existing directory instead of a file, it will
create "<url.PathEscape(pkgpath)>.mprof" in that directory.
The PathEscaped package names are ugly, but this puts all
the files in a single directory with no risk of name clashs,
which simplies the usual case for using these files, which
is something like
```
go tool pprof profiles/*.mprof
```
Creating a directory tree mimicking the package structure
requires something along the lines of
```
go tool pprof `find profiles -name "*.mprof" -print`
```
In addition, this turns off "legacy format" because that
is only useful for a benchcompile, which does not use this
new feature (and people actually interested in memory
profiles probably prefer the new ones).
Change-Id: Ic1d9da53af22ecdda17663e0d4bce7cdbcb54527
Reviewed-on: https://go-review.googlesource.com/c/go/+/539316
Run-TryBot: David Chase <drchase@google.com> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Keith Randall <khr@google.com>