Add new interface for the front end code to use when computing SHA1
checksums; the intent is to allow the different implementation in
different back ends.
No change in functionality for gccgo; this is an enabling change to
permit the front end to be used with other back ends (e.g. LLVM).
Reviewed-on: https://go-review.googlesource.com/28833
* go-sha1.cc: New file.
* Make-lang.in (GO_OBJS): Add go/go-sha1.o.
(CFLAGS-go/go-sha1.o): New variable.
From-SVN: r240053
