Consider using '-a' to get within-line coverage data

[MichaelChirico]

As intimated in #608 (comment), this looks like a pretty platform-robust way to get sub-line granularity of coverage (hence related to #604):

https://gcc.gnu.org/onlinedocs/gcc/Invoking-Gcov.html#Invoking-gcov

Write individual execution counts for every basic block. Normally gcov outputs execution counts only for the main blocks of a line. With this option you can determine if blocks within a single line are not being executed.

I'm not sure if this is complementary or at odds with #609, if at odds, #609 should still be preferred where possible since having structured data as the output will always be preferred to a text output we need to process manually.