Fix __errno for LP64 and clean up __get_tls.

If __get_tls has the right type, a lot of confusing casting can disappear.

It was probably a mistake that __get_tls was exposed as a function for mips
and x86 (but not arm), so let's (a) ensure that the __get_tls function
always matches the macro, (b) that we have the function for arm too, and
(c) that we don't have the function for any 64-bit architecture.

Change-Id: Ie9cb989b66e2006524ad7733eb6e1a65055463be
diff --git a/libc/bionic/pthread_create.cpp b/libc/bionic/pthread_create.cpp
index 63695d3..9e06afc 100644
--- a/libc/bionic/pthread_create.cpp
+++ b/libc/bionic/pthread_create.cpp
@@ -50,6 +50,8 @@
 
 extern "C" void ATTRIBUTES _thread_created_hook(pid_t thread_id);
 
+extern "C" int __set_tls(void* ptr);
+
 static const int kPthreadInitFailed = 1;
 
 static pthread_mutex_t gPthreadStackCreationLock = PTHREAD_MUTEX_INITIALIZER;