When gcov is supported in a cross-compiler setup, GCC assumes that
either the headers are present and shared with the host, or present
and set up in a sysroot, or absent. We have headers which aren’t
shared and aren’t in a sysroot, so we need to tell GCC where they are
*without* specifying them as an argument to --with-headers (the latter
must only be enabled).

Closes: #989862
LP: #1883933, #1920988