Valgrind's checking of uninitialised memory behaves very much like a
check for constant-time code: branches and memory indexes based on
uninitialised memory trigger warnings. Therefore, if we can tell
Valgrind that some secret is “uninitialised”, it'll give us a warning if
we do something non-constant-time with it.

This was the idea behind https://github.com/agl/ctgrind. But tricks like
that are no longer needed because Valgrind now comes with support for
marking regions of memory as defined or not. Therefore we can use that
API to check constant-time code.

This CL defines |CONSTTIME_SECRET| and |CONSTTIME_DECLASSIFY|, which are
no-ops unless the code is built with
|BORINGSSL_CONSTANT_TIME_VALIDATION| defined, which it isn't by default.
So this CL is a no-op itself so far. But it does show that a couple of
bits of constant-time time are, in fact, constant-time—seemingly even
when compiled with optimisations, which is nice.

The annotations in the RSA code are a) probably not marking all the
secrets as secret, and b) triggers warnings that are a little
interesting:

The anti-glitch check calls |BN_mod_exp_mont| which checks that the
input is less than the modulus. Of course, it is because the input is
the RSA plaintext that we just decrypted, but the plaintext is supposed
to be secret and so branching based on its contents isn't allows by
Valgrind. The answer isn't totally clear, but I've run out of time on
this for now.

Change-Id: I1608ed0b22d201e97595fafe46127159e02d5b1b
Reviewed-on: https://boringssl-review.googlesource.com/c/33504
Reviewed-by: Adam Langley <agl@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>