[sanitizer] Generalize FD closing in StartSubprocess (#192114)

Use internal_close_range with a fallback to the sysconf(_SC_OPEN_MAX)
loop. This removes the platform-specific #if and lets all platforms
benefit from close_range when supported.

Follow-up to #191971.

compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp

diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp
index 8e5e879..35b596d 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp
@@ -562,11 +562,10 @@
       internal_close(stderr_fd);
     }
 
-#  if SANITIZER_FREEBSD
-    internal_close_range(3, ~static_cast<fd_t>(0), 0);
-#  else
-    for (int fd = sysconf(_SC_OPEN_MAX); fd > 2; fd--) internal_close(fd);
-#  endif
+    // Close all fds except stdin/stdout/stderr before exec.
+    // Fallback to the loop if close_range is not supported.
+    if (internal_close_range(3, ~static_cast<fd_t>(0), 0) != 0)
+      for (int fd = sysconf(_SC_OPEN_MAX); fd > 2; fd--) internal_close(fd);
 
     internal_execve(program, const_cast<char **>(&argv[0]),
                     const_cast<char *const *>(envp));