go.tools/go/gccgoimporter: use gccgo -dumpversion
go.tools/go/gccgoimporter: use gccgo -dumpversion

GccgoInstallation.InitFromDriver currently parses
the output of gccgo -### to get the gcc version,
target triple, and library paths. At least with
Ubuntu's stock libgo5 package, the search path for
.gox files derived from the version is incorrect.

gccgo uses the DEFAULT_TARGET_VERSION macro when
constructing the search path; this value can be
retrieved from gccgo via the "-dumpversion" flag.

Fixes golang/go#7772.

LGTM=iant, gri
R=golang-codereviews, iant, gri
CC=golang-codereviews
https://golang.org/cl/88150043

jdhenke/tools,dmitris/tools,argandas/tools,magiccao/tools,springning/tools,cw2018/tools,jdhenke/tools,golang/tools,argandas/tools,matryer/tools,nicot/tools,maddyonline/tools,maddyonline/tools,josharian/tools,4ad/tools,dmitris/tools,josharian/tools,josharian/tools,maddyonline/tools,glasser/golang-tools,cmars/tools,argandas/tools,jdhenke/tools,syreclabs/go-tools,golang/tools,sshetty10/tools,cmars/tools,runcom/tools,syreclabs/go-tools,golang/tools,cw2018/tools,argandas/tools,4ad/tools,syreclabs/go-tools,magiccao/tools,syreclabs/go-tools,runcom/tools,runcom/tools,josharian/tools,nicot/tools,glasser/golang-tools,syreclabs/go-tools,dmitris/tools,sshetty10/tools,sshetty10/tools,cmars/tools,matryer/tools,springning/tools,golang/tools,nicot/tools,maddyonline/tools,4ad/tools,sshetty10/tools,cw2018/tools,cw2018/tools,syreclabs/go-tools,golang/tools,springning/tools,magiccao/tools,runcom/tools,magiccao/tools,springning/tools,dmitris/tools,matryer/tools,matryer/tools,cmars/tools,glasser/golang-tools,jdhenke/tools,glasser/golang-tools,nicot/tools