cmd/go: add -pgo build flag

cmd/go: add -pgo build flag

Add a -pgo flag for "go build" (and other build actions), to
specify the file path of a profile used for PGO. Special name
"off" turns off PGO.

The given profile path is passed to the compiler.

The build cache is sensitive to the content of the given PGO
profile.

TODO: auto mode.

For #55022.

Change-Id: Ieee1b131b4c041f9502fd0a1acf112f3e44246be
Reviewed-on: https://go-review.googlesource.com/c/go/+/438736
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>