crypto: sha512-avx2 - Fix RBP usage

Using RBP as a temporary register breaks frame pointer convention and
breaks stack traces when unwinding from an interrupt in the crypto code.

Mix things up a little bit to get rid of the RBP usage, without hurting
performance too much.  Use RDI instead of RBP for the TBL pointer.  That
will clobber CTX, so spill CTX onto the stack and use R12 to read it in
the outer loop.  R12 is used as a non-persistent temporary variable
elsewhere, so it's safe to use.

Also remove the unused y4 variable.

Reported-by: Eric Biggers <ebiggers3@gmail.com>
Reported-by: Peter Zijlstra <peterz@infradead.org>
Tested-by: Eric Biggers <ebiggers@google.com>
Acked-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>