The implementation we currently use from OpenSSL prevents the compiler
from optimizing away clensing operations on blocks of memory that are
about to be released, but this protection is not extended to link-time
optimization. This commit copies the solution cooked up by Google
compiler engineers which uses inline assembly directives to instruct the
compiler not to optimize out the call under any circumstances. As the
code is in-lined, this has the added advantage of removing one more
OpenSSL dependency.

Regarding license compatibility, Google's contributions to BoringSSL
library, including this code, is made available under the ISC license,
which is MIT compatible.

BoringSSL git commit: ad1907fe73334d6c696c8539646c21b11178f20f

ninja all check