cmd/go: add missing gccgo checks for buildmodeInit

Some recent failures in gccgo on linux/ppc64 identified
an error in buildmodeInit when buildmode=c-archive.
A fix went into gofrontend, and this is the
corresponding change for master. This change also includes
two other updates related to gccgo in this function that
were in the file from gofrontend but missing from master.

Updates #29046

Change-Id: I9a894e7d728e31fb9e9344cd61d50408df7faf4a
Reviewed-on: https://go-review.googlesource.com/c/152160


Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>