component/golang: build with slapos gcc

When ld version is too old (Debian 7 comes with version 2.2 which is too
old), golang testsuite fail with:

binutils_test.go:237: SourceLine for main: expect [{main /tmp/hello.c 3}]; got [{main  0}]

see also https://github.com/golang/go/issues/26584

We explicitly use gcc 8.2, but what we need is gcc > 5.5 because of
https://github.com/golang/go/issues/24046
When 8.2 (or newer) becomes the default SlapOS gcc, there should be no
problem in dropping the explicit version and using ${gcc:location} here.

component/golang/buildout.cfg

@@ -2,6 +2,7 @@
 [buildout]
 extends =
   ../findutils/buildout.cfg
+  ../gcc/buildout.cfg
   ../git/buildout.cfg
   ../pkgconfig/buildout.cfg
 
@@ -18,6 +19,7 @@ make-targets= cd src && ./all.bash && cp -alf .. ${:location}
 # some testdata files have an issue with slapos.extension.strip.
 post-install = ${findutils:location}/bin/find ${:location}/src -type d -name testdata -exec rm -rf {} \; || true
 environment =
+  PATH=${gcc-8.2:location}/bin:%(PATH)s
   GOROOT_FINAL=${:location}
   ${:environment-extra}