Now the only allocations in ECDSA are the ECDSA_SIG input and output.

Change-Id: If1fcde6dc2ee2c53f5adc16a7f692e22e9c238de
Reviewed-on: https://boringssl-review.googlesource.com/c/33069
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>