We build with them for C files, so include it for the assembly files
too.

Change-Id: I1c595e18314b245330a6f047d0a97a031d55b115
Reviewed-on: https://boringssl-review.googlesource.com/9150
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

Uses LCOV for C(++) line coverage and Valgrind's Callgrind tool to
generate assembly-level line coverage for the generated assembly
code.

BUG=590332

Change-Id: Ic70300a272c38f4fa6dd615747db568aa0853584
Reviewed-on: https://boringssl-review.googlesource.com/7251
Reviewed-by: David Benjamin <davidben@google.com>