ci: update and improve FreeBSD CI

[lcheylus]

Now that all the tests have been fixed for FreeBSD with tcc, I propose to update and improve "FreeBSD CI":

• use cross-platform-actions/action GH action instead of vmactions/freebsd-vm action. This action is more reliable and runs faster on amd64 architecture. (< 10 minutes to run all tests with gcc)
• use a FreeBSD VM on amd64 arch with 14.2 version (14.3 is not available for now, see Add support for FreeBSD 14.3 cross-platform-actions/action#106).
• install libiconv and boehm-gc-threaded packages for build/tests on FreeBSD
• use gmake (GNU make) instead of BSD make to build V => tcc is detected during build
• remove unset of CFLAGS for V build
• split this workflow with 3 jobs (using different value of VFLAGS for each):
  • one for tests with tcc
  • one for tests with clang (version 19.1.7)
  • one for tests with gcc (version 13.3.0)
• All tests run with script ci/freebsd_ci.vsh (unmodified)

✅ Example of this "FreeBSD CI" workflow in my own repository => https://github.com/lcheylus/v/actions/runs/15663998010

[spytheman]

Excellent work. It is indeed much faster now.