Matched gcov to the compiler that produced the coverage data

[fdesbiens]

Companion to #656 and #657. Neither is wrong; together they open a hole that this closes.

gcov reads a data format tied to the compiler that produced it. coverage.sh took whatever gcov was first on PATH, which was fine while the compiler was also whatever was first on PATH. #656 made cmake/linux.cmake honour CC, and #657 made a compiler switch actually reconfigure the build — so that assumption no longer holds, and the first person to use the new capability would have hit this.

Measured on dev with both merged:

$ CC=gcc-14 ./run.sh build default_build_coverage     # succeeds
$ ./coverage.sh default_build_coverage
RuntimeError: GCOV returncode was 3.
EXIT=64

gcov says why, if asked directly:

tx_block_allocate.c.gcno:version 'B42*', prefer 'B33*'

The part that would have cost someone an afternoon is where it surfaces: the tests all pass, then the coverage step dies with a Python traceback, which reads like a gcovr bug rather than a toolchain mismatch.

The change

gcov is derived from CC instead of found on PATH, so the caller sets one variable rather than remembering two. GCOV still overrides, for a toolchain that does not follow the gcc/gcov naming, and a derived gcov that does not exist is reported as such rather than surfacing as a traceback.

Applied to both test/tx/cmake/coverage.sh and test/smp/cmake/coverage.sh.

Verified, both suites

Invocation	Before	After
CC=gcc-14	exit 64, traceback	exit 0 — 177 files, 1527/3827 lines
CC unset (what CI does)	exit 0	exit 0 — 177 files, 1527/3827 lines, unchanged
CC=gcc-99	n/a	exit 1, naming gcov-99 and CC, not a traceback
GCOV=gcov-14 with CC=gcc-99	n/a	exit 0 — the override still wins

CI is unaffected: nothing in the workflows sets CC, so the default path is byte-for-byte the behaviour it had before.

One deliberate non-fix

Reading a gcc-14 tree with the default gcc-13 gcov still fails with exit 64. That pairing is wrong, and producing a number from mismatched coverage data would be worse than refusing. Only the case where CC is stated is made to work.