Run test262 tests in parallel (#564)

This commit introduces a couple of changes in order to make run-test262
go brr and execute tests in parallel:

- Remove CONFIG_AGENT build option. The disabled version of the build
  was already broken and no one noticed, Remove the define altogether.

- Remove the -C switch. Hard to support in multi-threaded mode.
  I may bring it back some day because it _is_ useful.

- Remove the -r switch. Also hard to support and I never look at
  test262_report.txt anyway so on the chopping block it goes.

- Judicious use of thread-local storage so I don't have to thread
  through state everywhere and embiggen the diff even more.

This is what Real Programmers(TM) do: stay up coding way past midnight
just so the test suite finishes in one minute instead of four.

Fixes: #547

CMakeLists.txt

@@ -269,8 +269,15 @@ endif()
 # Test262 runner
 #
 
-# run-test262 uses pthreads.
-if(NOT WIN32 AND NOT EMSCRIPTEN)
+if(WIN32
+OR EMSCRIPTEN
+OR CMAKE_C_COMPILER_ID STREQUAL "TinyCC"
+OR CMAKE_C_COMPILER_ID STREQUAL "GNU" AND CMAKE_C_COMPILER_VERSION VERSION_LESS 5)
+    # Empty. run-test262 uses pthreads, sorry Windows users.
+    # tcc and gcc 4.8 don't understand _Thread_local, whereas I
+    # don't understand why people still use 4.8 in this day and age
+    # but hey, here we are.
+else()
     add_executable(run-test262
         run-test262.c
     )