TLS (GNU/C11 thread-local storage) support for static-linked programs

the design for TLS in dynamic-linked programs is mostly complete too,
but I have not yet implemented it. cost is nonzero but still low for
programs which do not use TLS and/or do not use threads (a few hundred
bytes of new code, plus dependency on memcpy). i believe it can be
made smaller at some point by merging __init_tls and __init_security
into __libc_start_main and avoiding duplicate auxv-parsing code.

at the same time, I've also slightly changed the logic pthread_create
uses to allocate guard pages to ensure that guard pages are not
counted towards commit charge.

src/internal/libc.h

@@ -16,6 +16,7 @@ struct __libc {
 	int canceldisable;
 	FILE *ofl_head;
 	int ofl_lock[2];
+	volatile size_t tls_size, tls_cnt;
 };
 
 extern size_t __hwcap;