cmd/link: added test of availability of gcc

cmd/link: added test of availability of gcc

The test RuntimeTypeAttr always failed when gcc
was unavailable. The test is duplicated for internal
and external linking. The usual verification
of host linker is added at the beginning of the
external link test.

Fixes #26621

Change-Id: I076d661f854c8a6de8fa5e7b069942a471445047
Reviewed-on: https://go-review.googlesource.com/126075
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>