[dev.boringcrypto] crypto/internal/boring: allow hmac operations after Sum

[dev.boringcrypto] crypto/internal/boring: allow hmac operations after Sum

hmac.New returns a hash.Hash, which defines Sum as:

	// Sum appends the current hash to b and returns the resulting slice.
	// It does not change the underlying hash state.
	Sum(b []byte) []byte

I've now seen two different pieces of code that make
use of the assumption that Sum has no effect on the
internal state, so make it so.

Test in next CL in stack, so that it can be cherry-picked
to master.

Change-Id: Iad84ab3e2cc12dbecef25c3fc8f2662d157b0d0b
Reviewed-on: https://go-review.googlesource.com/63910
Reviewed-by: Adam Langley <agl@golang.org>