feat: exec Fortran compiles via argv (posix_spawn/CreateProcess)

[krystophny]

Summary

• Execute the Fortran compile command through a direct argv spawn instead of
  a /bin/sh -c (or cmd /c) string, on both Unix and Windows
• posix_spawnp (Unix) and CreateProcess (Windows) are the
  async/multithread-safe replacements for fork+exec
• If the spawn is unavailable or fails, falls back to the existing shell
  run(), so this can only degrade to prior behavior, never break the build
• Bootstrap builds (no C) stay on the shell path

Why this is a safety fix, not just a speedup

execute_command_line uses system(3), which forks, and concurrent fork from
a multithreaded (OpenMP) process is documented non-thread-safe. Commit
bb891e3f records a real observed failure (ar argument truncation on macOS-14
gcc 13 in CI) and works around it by wrapping link/archive in
critical(run_command), noting the compile path carries the same latent race.

Spawning the compiler by argv removes that race on the compile path and removes
the shell entirely, so quoting/escaping fragility and the shell command-length
ceiling go away too.

Merge order

This is PR 1 of 3 in the argv spawn chain. Merge in order:

1. This PR — Fortran compiles via argv
2. Parallel link/archive via argv (feat: run link and archive in parallel via argv spawn #1300)
3. C/C++ compiles via argv (feat: exec C/C++ compiles via argv, close the shell-fork gap #1301)

Refs: #962

Verification

Unix, OpenMP active:

$ fpm build --flag -fopenmp
[100%] Project compiled successfully.

$ fpm test --flag -fopenmp
TALLY=TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT
TALLY=TTTTTTTTTTTT
TALLY;TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT
PASSED: all 35 tests passed

A freshly built binary builds and runs a sample project through the argv path.
The C file cross-compiles cleanly with x86_64-w64-mingw32-gcc -Wall -Wextra.