Providing a size hint to the allocator is substantially faster,
especially as we already know/need the size for OPENSSL_cleanse.
We provide a weak symbol that falls back to free when a malloc with
sdallocx is not statically linked with BoringSSL.
Alternatives considered:
* Use dlsym(): This is prone to fail on statically linked binaries
without symbols. Additionally, the extra indirection adds call
overhead above and beyond the linker resolved technique we're using.
* Use CMake rules to identify whether sdallocx is available: Once the
library is built, we may link against a variety of malloc
implementations (not all of which may have sdallocx), so we need to
have a fallback when the symbol is unavailable.
Change-Id: I3a78e88fac5b6e5d4712aa0347d2ba6b43046e07
Reviewed-on: https://boringssl-review.googlesource.com/31784
Reviewed-by: Chris Kennelly <ckennelly@google.com>
Reviewed-by: Adam Langley <agl@google.com>