cmd/cgo: don't emit #pragma declarations when using gccgo
cmd/cgo: don't emit #pragma declarations when using gccgo

Update issue 5905

Don't emit #pragma cgo_ldflag and cgo_import_static
declarations that gccgo doesn't understand and can't use.

R=golang-dev, rsc, bradfitz
CC=golang-dev
https://codereview.appspot.com/11490043

tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go