errno and *_init cleanup

errno and *_init cleanup

Define a common view of __errno_location, __h_errno_location in common header
and use that everywhere, __uClibc_main.c is no special.
The rule adopted:
for enabled threads we make in libc the __GI_x() variants strong, x() weak
and (should) provide another strong x() in libpthread.
If threads are disabled, even the __GI_x() variants are weak.

_stdio_init,_stdio_term,_locale_init: make all hidden weak in common header

Signed-off-by: Peter S. Mazinger <ps.m@gmx.net>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>