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

Use R12 instead of RBP.  R12 can't be used as the RT0 register because
of x86 instruction encoding limitations.  So use R12 for CTX and RDI for
CTX.  This means that CTX is no longer an implicit function argument.
Instead it needs to be explicitly copied from RDI.
Reported-by: NEric Biggers <ebiggers@google.com>
Reported-by: NPeter Zijlstra <peterz@infradead.org>
Tested-by: NEric Biggers <ebiggers@google.com>
Acked-by: NEric Biggers <ebiggers@google.com>
Signed-off-by: NJosh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>