Fix the C library initialization to avoid calling static C++ constructors twice.

The problem was due to the fact that, in the case of dynamic executables,
the dynamic linker calls the DT_PREINIT_ARRAY, DT_INIT and DT_INIT_ARRAY
constructors when loading shared libraries and dynamic executables,
*before* calling the executable's entry point (i.e. arch-$ARCH/bionic/crtbegin_dynamic.c)
which in turns call __libc_init() in libc.so, as defined by bionic/libc_init_dynamic.c

The latter did call these constructors array again, mistakenly.

The patch also updates the documentation of many related functions.

Also adds a new section to linker/README.TXT explaining restrictions on
C library usage.

The patch has been tested on a Dream for stability issues with
proprietary blobs:

- H264 decoding works
- Camera + Video recording works
- GPS works
- Sensors work

The tests in system/extra/tests/bionic/libc/common/test_static_cpp_mutex.cpp has been
run and shows the static C++ constructor being called only once.

linker/linker.c

@@ -44,7 +44,7 @@
 #include <sys/atomics.h>
 
 /* special private C library header - see Android.mk */
-#include <bionic_tls.h>
+#include <bionic_preinit.h>
 
 #include "linker.h"
 #include "linker_debug.h"
@@ -1691,6 +1691,7 @@ int main(int argc, char **argv)
 #define ANDROID_TLS_SLOTS  BIONIC_TLS_SLOTS
 
 static void * __tls_area[ANDROID_TLS_SLOTS];
+static pthread_internal_t  __main_thread;
 
 unsigned __linker_init(unsigned **elfdata)
 {
@@ -1709,8 +1710,7 @@ unsigned __linker_init(unsigned **elfdata)
     gettimeofday(&t0, 0);
 #endif
 
-    __set_tls(__tls_area);
-    ((unsigned *)__get_tls())[TLS_SLOT_THREAD_ID] = gettid();
+    __libc_preinit(&__main_thread, __tls_area);
 
     debugger_init();