cmd/compile: clean the output of GOSSAFUNC

Since we print almost everything to ssa.html in the GOSSAFUNC mode,
there is a need to stop spamming stdout when user just wants to see
ssa.html.

This changes cleans output of the GOSSAFUNC debug mode.
To enable the dump of the debug data to stdout, one must
put suffix + after the function name like that:

GOSSAFUNC=Foo+

Otherwise gc will not print the IR and ASM to stdout after each phase.
AST IR is still sent to stdout because it is not included
into ssa.html. It will be fixed in a separate change.

The change adds printing out the full path to the ssa.html file.

Updates #25942

Change-Id: I711e145e05f0443c7df5459ca528dced273a62ee
Reviewed-on: https://go-review.googlesource.com/126603


Run-TryBot: Yury Smolsky <yury@smolsky.by>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>