libc: Fix 'index' symbol export.

The C library didn't export the 'index' symbol, but its C++ name-mangling
instead, which broke the ABI and prevented some applications from loading
properly.

The main reason was that the implementation under bionic/index.cpp relied
on the declaration to specify that the function has C linkage.

However, the declaration for index() was removed from both <string.h>
and <strings.h> in a recent patch, which made the compiler think it was
ok to compile the function with C++ linkage instead!

This patch does the following:

- Move index() definition to bionic/ndk_cruft.cpp and ensure it uses
  C linkage.

  Note that this removes index() from the 64-bit library entirely, this
  is intentional and will break source compatibility. Simply replacing
  an index() call with the equivalent strchr() should be enough to fix
  this in third-party code.

- Remove bionic/index.cpp from the tree and build files.

- Remove x86 assembly implementation from arch-x86/ to avoid conflict
  with the one in ndk_cruft.cpp

BUG=15606653

Change-Id: I816b589f69c8f8a6511f6be6195d20cf1c4e8123

libc/arch-arm/arm.mk

@@ -12,7 +12,6 @@ libc_bionic_src_files_arm := \
     bionic/mmap.cpp
 
 libc_common_src_files_arm += \
-    bionic/index.cpp \
     bionic/memchr.c \
     bionic/memrchr.c \
     bionic/strchr.cpp \