[Support] Implement getMainExecutable for Haiku

This uses argv[0] on Haiku.

Author: brad0

llvm/lib/Support/Unix/Path.inc

@@ -129,7 +129,7 @@ const file_t kInvalidFile = -1;
 #if defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) ||      \
     defined(__FreeBSD_kernel__) || defined(__linux__) || defined(__CYGWIN__) || \
     defined(__DragonFly__) || defined(_AIX) || defined(__GNU__) ||              \
-    (defined(__sun__) && defined(__svr4__))
+    (defined(__sun__) && defined(__svr4__) || defined(__HAIKU__))
 static int test_dir(char ret[PATH_MAX], const char *dir, const char *bin) {
   struct stat sb;
   char fullpath[PATH_MAX];
@@ -283,7 +283,7 @@ std::string getMainExecutable(const char *argv0, void *MainAddr) {
   // Fall back to the classical detection.
   if (getprogpath(exe_path, argv0))
     return exe_path;
-#elif defined(__OpenBSD__)
+#elif defined(__OpenBSD__) || defined(__HAIKU__)
   char exe_path[PATH_MAX];
   // argv[0] only
   if (getprogpath(exe_path, argv0) != NULL)