dashboard: run freebsd 386 builder with GOHOSTARCH=386
dashboard: run freebsd 386 builder with GOHOSTARCH=386

The freebsd 386 builder is run on a FreeBSD amd64 host - without
GOHOSTARCH=386 this is generating a 64-bit tool chain with a 32-bit
runtime and disabling cgo. The previous issue with this configuration
is fixed with https://golang.org/cl/18420.

Fixes #13873.

Change-Id: I3ceef59055c5409773eab06f3980277854c61e89
Reviewed-on: https://go-review.googlesource.com/18421
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

proppy/build,golang/build,golang/build,proppy/build,proppy/build,golang/build,golang/build,proppy/build