Fix llvm::sys::fs::getMainExecutable on Hurd

[pinotree]

Summary

Use the getprogpath way on Hurd too, so getMainExecutable can properly
work, and thus find other llvm/clang tools.

lib/Support/Unix/Path.inc

 using namespace llvm;
 
 namespace llvm {
 namespace sys  {
 namespace fs {
 #if defined(__FreeBSD__) || defined (__NetBSD__) || defined(__Bitrig__) || \
     defined(__OpenBSD__) || defined(__minix) || defined(__FreeBSD_kernel__) || \
     defined(__linux__) || defined(__CYGWIN__) || defined(__DragonFly__) || \
-    defined(_AIX)
+    defined(_AIX) || defined(__GNU__)
 static int
 test_dir(char ret[PATH_MAX], const char *dir, const char *bin)
 {
   struct stat sb;
   char fullpath[PATH_MAX];
 
   snprintf(fullpath, PATH_MAX, "%s/%s", dir, bin);
   if (!realpath(fullpath, ret))
#if defined(__APPLE__)
   uint32_t size = sizeof(exe_path);
   if (_NSGetExecutablePath(exe_path, &size) == 0) {
     char link_path[MAXPATHLEN];
     if (realpath(exe_path, link_path))
       return link_path;
   }
 #elif defined(__FreeBSD__) || defined (__NetBSD__) || defined(__Bitrig__) || \
       defined(__OpenBSD__) || defined(__minix) || defined(__DragonFly__) || \
-      defined(__FreeBSD_kernel__) || defined(_AIX)
+      defined(__FreeBSD_kernel__) || defined(_AIX) || defined(__GNU__)
   char exe_path[PATH_MAX];
 
   if (getprogpath(exe_path, argv0) != NULL)
     return exe_path;
 #elif defined(__linux__) || defined(__CYGWIN__)
   char exe_path[MAXPATHLEN];
   StringRef aPath("/proc/self/exe");
   if (sys::fs::exists(aPath)) {