go-webrtc: fixes for 386

In preparing a Gitian build of Snowflake for linux-386, I ran into architecture problems that are fixed by the attached patch.

The first just adds a webrtc-linux-386.pc, like already existed for linux-amd64 and darwin-amd64.

The second reduces the size of some static Cgo array bounds, to avoid this error:

configuration.go:228[/tmp/go-build928087590/github.com/keroserene/go-webrtc/_obj/configuration.cgo1.go:238]: type [1073741824]*_Ctype_char larger than address space