Improve `CommandLine.executablePath` implementation on OpenBSD

[grynspan]

Currently on OpenBSD, our implementation of CommandLine.executablePath just takes argv[0] verbatim. This works poorly if a relative path is used to start the process (e.g. ./foo). But we can improve it somewhat by calling realpath() as early as possible during execution. Something like:

#include <link.h>
#include <atomic>

static std::atomic<char *> executablePath = nullptr;

__attribute__((__constructor__(101)))
static void tryResolvingExecutablePath(void) {
  (void)dl_iterate_phdr([] (struct dl_phdr_info *info, size_t size, void *cookie) -> int {
    char *path = nullptr;

    if (info->dlpi_name) {
      if (strchr(info->dlpi_name, '/')) {
        path = realpath(info->dlpi_name, nullptr);
      }
      if (!path) {
        path = strdup(info->dlpi_name);
      }
      // NOTE: we do not attempt to resolve $PATH-relative paths here because we don't expect test executables to be installed into e.g. /usr/local/bin.
    }
    executablePath.store(path);

    return 1; // we only care about the first image
  }, nullptr);
}

extern "C" char *swt_getExecutablePath(void);
char *swt_getExecutablePath(void) {
  return executablePath.load();
}

We would need to use a constructor function to ensure this code runs as early as possible to minimize the risk of somebody changing the current working directory and breaking the relative path.

[grynspan]

@3405691582 Any thoughts?