[dev.boringcrypto] misc/boring: add go1.12.1b4 and update build scripts
The inliner seems to have gotten a bit too smart in 1.12 and it made
sha1.boringNewSHA1 disappear. Replace it with the proper
crypto/internal/boring/sig.BoringCrypto signature. Also, switch the
negative signature to sha256.(*digest), since SHA-256 is used for sure
by cmd/go. Not using crypto/internal/boring/sig.StandardCrypto just to
be safe, in case the crypto/internal/boring/sig mechanism breaks.
Also, had to fight #30833 and #30515 to get
golang.org/x/build/cmd/release to build in modules mode.
Change-Id: I46f1471582fd77daae47d00baab975109902052d
Reviewed-on: https://go-review.googlesource.com/c/go/+/169517 Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>