[libc++][test] Fix infinite loop when mkstemp fails

mkstemp is guaranteed to make at least TMP_MAX attempts to create the
random file, and if it can't, it fails with EEXIST. get_temp_file_name
shouldn't call mkstemp again if it fails with anything other than
EEXIST. A single mkstemp call seems sufficient.

On Android, I've seen mkstemp fail with:
 - EROFS (because cwd wasn't set to a writable filesystem)
 - EACCES (because cwd pointed to a dir owned by root, but the test
   program was running as the shell user instead)

Previously, get_temp_file_name would run forever in these situations.

See D4962 and "llvm-svn: 229035"

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D130214

libcxx/test/support/platform_support.h

@@ -75,16 +75,12 @@ std::string get_temp_file_name()
         abort();
     }
 #else
-    std::string Name;
-    int FD = -1;
-    do {
-        Name = "libcxx.XXXXXX";
-        FD = mkstemp(&Name[0]);
-        if (FD == -1 && errno == EINVAL) {
-            perror("mkstemp");
-            abort();
-        }
-    } while (FD == -1);
+    std::string Name = "libcxx.XXXXXX";
+    int FD = mkstemp(&Name[0]);
+    if (FD == -1) {
+        perror("mkstemp");
+        abort();
+    }
     close(FD);
     return Name;
 #endif