cmd/go: include Fortran files in build ID computation
cmd/go: include Fortran files in build ID computation

Otherwise removing a .f file won't trigger a rebuild.
Noticed by inspection while working on the code.
I don't have a good way to write a test for this,
and I expect the code to change significantly in the next
release, but might as well get it right for Go 1.9.

Change-Id: I3f6f9f71b3a7d4f0be49a47419dac59899959e7c
Reviewed-on: https://go-review.googlesource.com/44855
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

josharian/go,nawawi/go,nawawi/go,vsekhar/elastic-go,josharian/go,cloudflare/go,vsekhar/elastic-go,akutz/go,codestation/go,CAFxX/go,polyverse-security/go,polyverse-security/go,nawawi/go,cloudflare/go,josharian/go,ImJasonH/go,cloudflare/go,ImJasonH/go,christopher-henderson/Go,CAFxX/go,polyverse-security/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,josharian/go,akutz/go,vsekhar/elastic-go,josharian/go,vsekhar/elastic-go,nawawi/go,polyverse-security/go,CAFxX/go,codestation/go,akutz/go,polyverse-security/go,golang/go,cloudflare/go,akutz/go,cloudflare/go,golang/go,ImJasonH/go,codestation/go,ImJasonH/go,cloudflare/go,ImJasonH/go,codestation/go,Cofyc/go,vsekhar/elastic-go,josharian/go,codestation/go,golang/go,codestation/go,ImJasonH/go,josharian/go,vsekhar/elastic-go,golang/go,Cofyc/go,Cofyc/go,codestation/go,nawawi/go,polyverse-security/go,nawawi/go,ImJasonH/go,nawawi/go,codestation/go,polyverse-security/go,Cofyc/go,Cofyc/go,akutz/go,akutz/go,vsekhar/elastic-go,CAFxX/go,polyverse-security/go,CAFxX/go,akutz/go,Cofyc/go,cloudflare/go,vsekhar/elastic-go,Cofyc/go,ImJasonH/go,CAFxX/go,golang/go,akutz/go,CAFxX/go,josharian/go,Cofyc/go,golang/go