compiler: add abstraction layer for sha1 checksums.

compiler: add abstraction layer for sha1 checksums.

    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