unify static and dynamic linked implementations of thread-local storage

this both allows removal of some of the main remaining uses of the
SHARED macro and clears one obstacle to static-linked dlopen support,
which may be added at some point in the future.

specialized single-TLS-module versions of __copy_tls and __reset_tls
are removed and replaced with code adapted from their dynamic-linked
versions, capable of operating on a whole chain of TLS modules, and
use of the dynamic linker's DSO chain (which contains large struct dso
objects) by these functions is replaced with a new chain of struct
tls_module objects containing only the information needed for
implementing TLS. this may also yield some performance benefit
initializing TLS for a new thread when a large number of modules
without TLS have been loaded, since since there is no need to walk
structures for modules without TLS.

src/internal/libc.h

@@ -11,13 +11,20 @@ struct __locale_struct {
 	const struct __locale_map *volatile cat[6];
 };
 
+struct tls_module {
+	struct tls_module *next;
+	void *image;
+	size_t len, size, align, offset;
+};
+
 struct __libc {
 	int can_do_threads;
 	int threaded;
 	int secure;
 	volatile int threads_minus_1;
 	size_t *auxv;
-	size_t tls_size;
+	struct tls_module *tls_head;
+	size_t tls_size, tls_align, tls_cnt;
 	size_t page_size;
 	struct __locale_struct global_locale;
 };