Fix weak/strong attribute of __errno_location and it's __GI alias

Fix weak/strong attribute of __errno_location and it's __GI alias

A simple statically linked hello world program was segfaulting for ARC
in linuxthreads.old configuration (although the root casue applies
cross-arch for NPTL as well as linuxthreads.old as described)

The crash was due to branch to NULL in _stdio_init

  0001026c <_stdio_init>:
     1026c:    push_s     blink
     1026e:    st.a       r13,[sp,-8]
     10272:    bl.d       0  --> supposed call to __errno_location

The call was NOT getting patched to libc internal only alias
__GI___errno_location, because it was weak while it's exported cousin,
__errno_location was strong/normal.

  arc-linux-uclibc-nm libc/misc/internals/__errno_location.os

  00000000 W __GI___errno_location
  00000000 T __errno_location

This is exactly opposite to what is expected.

Quoting Peter S. Mazinger, commit 87936cd013041 "errno and *_init cleanup"

 | 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.

With the fix, we see the right settings as below

  00000000 T __GI___errno_location
  00000000 W __errno_location

Note that problem won't show up in a static busybox build as it references
errno and that seems to elide the issue.

I can confirm the same/more issues with latest ARM buildroot builds w/o
my fix.

(1). linuxthreads.old (broken just like ARC)

  arm-linux-nm uclibc-snapshot/libc/misc/internals/__errno_location.os

  00000000 W __GI___errno_location
  00000000 T __errno_location

  But presumably the issue there is NOT catestrophic because ARM linker is
  likely smarter and patches a NOP instead of NULL branch.

  00008388 <_stdio_init>:
      8388:	e92d4038 	push	{r3, r4, r5, lr}
      838c:	e320f000 	nop	{0}

(2) NPTL build (exported version is not weak)

  00000000 T __GI___errno_location
  00000000 T __errno_location

  This causes a static link with libpthread and test program
  referencing errno to fail to link.

  #include <errno.h>
  int main(void)
  {
      printf("%d\n", errno);
  }

  arm-linux-gcc -static -pthread -o tst tst.o

  arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libc.a(__errno_location.os):
  In function `__errno_location':  __errno_location.c:(.text+0x0):
                           multiple definition of `__errno_location'
  arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libpthread.a
            (errno_location.os):errno_location.c:(.text+0x0): first defined here

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>