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>