Make _Thread_local work across platforms

We now rewrite the binary image at runtime on Windows and XNU to change
mov %fs:0,%reg instructions to use %gs instead. There's also simpler
threading API introduced by this change and it's called _spawn() and
_join(), which has replaced most clone() usage.

test/libc/stdio/test.mk

@@ -37,6 +37,7 @@ TEST_LIBC_STDIO_DIRECTDEPS =				\
 	LIBC_SYSV					\
 	LIBC_TINYMATH					\
 	LIBC_TESTLIB					\
+	LIBC_THREAD					\
 	LIBC_TIME					\
 	LIBC_LOG					\
 	LIBC_UNICODE					\