cmd/cgo: remove //extern for check functions in gccgo Go prologue
cmd/cgo: remove //extern for check functions in gccgo Go prologue

The //extern comments are incorrect and cause undefined symbol
errorswhen building cgo code with -compiler=gccgo. The code is already
designed to use weak references, and that support relies on the cgo
check functions being treated as local functions.

Change-Id: Ib38a640cc4ce6eba74cfbf41ba7147ec88769ec0
Reviewed-on: https://go-review.googlesource.com/23014
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

cloudflare/go,codestation/go,christopher-henderson/Go,josharian/go,CAFxX/go,cloudflare/go,suncycheng/go,vsekhar/elastic-go,4ad/go,nawawi/go,Cofyc/go,josharian/go,josharian/go,momchil-velikov/go,golang/go,cloudflare/go,4ad/go,cloudflare/go,4ad/go,vsekhar/elastic-go,ganboing/go-esx,akutz/go,derekmarcotte/go,golang/go,ganboing/go-esx,ImJasonH/go,akutz/go,ImJasonH/go,vsekhar/elastic-go,4ad/go,polyverse-security/go,derekmarcotte/go,4ad/go,theif519/go,cloudflare/go,akutz/go,polyverse-security/go,codestation/go,linux-on-ibm-z/go,nawawi/go,momchil-velikov/go,vsekhar/elastic-go,codestation/go,linux-on-ibm-z/go,akutz/go,golang/go,akutz/go,akutz/go,CAFxX/go,linux-on-ibm-z/go,polyverse-security/go,CAFxX/go,cloudflare/go,ganboing/go-esx,ImJasonH/go,suncycheng/go,linux-on-ibm-z/go,derekmarcotte/go,theif519/go,codestation/go,derekmarcotte/go,Cofyc/go,nawawi/go,momchil-velikov/go,CAFxX/go,goodlang/good,cloudflare/go,ganboing/go-esx,theif519/go,theif519/go,vsekhar/elastic-go,goodlang/good,golang/go,goodlang/good,codestation/go,cloudflare/go,derekmarcotte/go,josharian/go,CAFxX/go,CAFxX/go,CAFxX/go,akutz/go,vsekhar/elastic-go,nawawi/go,polyverse-security/go,momchil-velikov/go,suncycheng/go,Cofyc/go,nawawi/go,theif519/go,ganboing/go-esx,derekmarcotte/go,vsekhar/elastic-go,derekmarcotte/go,josharian/go,nawawi/go,4ad/go,golang/go,golang/go,suncycheng/go,akutz/go,codestation/go,theif519/go,codestation/go,theif519/go,ImJasonH/go,CAFxX/go,polyverse-security/go,vsekhar/elastic-go,theif519/go,ganboing/go-esx,josharian/go,momchil-velikov/go,ganboing/go-esx,linux-on-ibm-z/go,ImJasonH/go,4ad/go,momchil-velikov/go,ImJasonH/go,suncycheng/go,Cofyc/go,ImJasonH/go,linux-on-ibm-z/go,suncycheng/go,polyverse-security/go,goodlang/good,4ad/go,josharian/go,goodlang/good,josharian/go,Cofyc/go,momchil-velikov/go,nawawi/go,Cofyc/go,goodlang/good,polyverse-security/go,Cofyc/go,ganboing/go-esx,momchil-velikov/go,nawawi/go,linux-on-ibm-z/go,derekmarcotte/go,golang/go,linux-on-ibm-z/go,goodlang/good,suncycheng/go,polyverse-security/go,goodlang/good,golang/go,ImJasonH/go,Cofyc/go,codestation/go,suncycheng/go