[dev.boringcrypto] crypto/internal/boring: make SHA calls allocation-free

[dev.boringcrypto] crypto/internal/boring: make SHA calls allocation-free

The standard Go implementations are allocation-free.
Making the BoringCrypto ones the same helps avoid
surprises, including in some of our own tests.

For #51940.

Change-Id: Ic9c5dc46f5e29ca85f571244be2b380ec2cf89c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/395876
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>