coverage: add a wrapper for execveat()

coverage: add a wrapper for execveat()

gcov provides wrappers for the exec*() calls but there's none for execveat(),
which means we lose all coverage prior to the call. To mitigate this, let's
add a simple execveat() wrapper in gcov's style[0], which dumps and resets
the coverage data when needed.

This applies only when we're built with -Dfexecve=true.

[0] https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libgcc/libgcov-interface.c;h=b2ee930864183b78c8826255183ca86e15e21ded;hb=HEAD