This change was made to the gccgo sources as part of CL 47037.
It is required to make the testcarchive and testcshared tests work.
Otherwise using `go build -mode=c-archive -o libgo.a` will cause the
header file to be named go.h rather than libgo.h.

Change-Id: I2db1d7b0f575368b31273cc01097447a0471efd6
Reviewed-on: https://go-review.googlesource.com/111615
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>