cmd/go: always use --whole-archive for gccgo packages

This is, in effect, what the gc toolchain does.  It fixes cases where Go
code refers to a C global variable; without this, if the global variable
was the only thing visible in the C code, the generated cgo file might
not get pulled in from the archive, leaving the Go variable
uninitialized.

This was reported against gccgo as https://gcc.gnu.org/PR68255 .

Change-Id: I3e769dd174f64050ebbff268fbbf5e6fab1e2a1b
Reviewed-on: https://go-review.googlesource.com/16775


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>