This extends the cgo changes in http://golang.org/cl/8094 to gccgo.
It also adds support for setting runtime_iscgo correctly for gccgo;
the gc runtime bases the variable on the runtime/cgo package, but
gccgo has no equivalent to that package.

The go tool supports -buildmode=c-archive for gccgo by linking all the
Go objects together using -r.  For convenience this object is then put
into an archive file.

The go tool now passes -fsplit-stack when building C code for gccgo on
386 and amd64.  This is required for using -r and will also cut down
on unnecessary stack splits.

The go tool no longer applies standard package cgo LDFLAGS when using
gccgo.  This is mainly to avoid getting confused by the LDFLAGS in the
runtime/cgo package that gccgo does not use.

Change-Id: I1d0865b2a362818a033ca9e9e901d0ce250784e7
Reviewed-on: https://go-review.googlesource.com/9511Reviewed-by: David Crawshaw <crawshaw@golang.org>