use GNU C89 extern inline semantics for __size_mul_overflow()

use GNU C89 extern inline semantics for __size_mul_overflow()

__size_mul_overflow generates warning under following compilation envrionment:

-OX -D_FORTIFY_SOURCE=2 (X=1, 2, 3)

For example:

echo '#include <stdio.h>' | \
prebuilts/gcc/linux-x86/arm/arm-linux-androideabi-4.9/bin/arm-linux-androideabi-gcc \
    -I bionic/libc/arch-arm/include \
    -I bionic/libc/include \
    -I bionic/libc/kernel/uapi \
    -I bionic/libc/kernel/common \
    -I bionic/libc/kernel/uapi/asm-arm \
    -I bionic/libm/include \
    -I bionic/libm/include/arm \
    -I bionic/libc/include \
    -Werror \
    -O1 \
    -D_FORTIFY_SOURCE=2 \
    -c \
    -x c \
    -

bionic/libc/include/stdio.h:360:13: error: '__size_mul_overflow' is
static but used in inline function 'fread' which is not static [-Werror]
         if (__size_mul_overflow(size, count, &total)) {
                          ^

C99 - 6.7.4
"An inline definition of a function with external linkage shall not contain
a definition of a modifiable object with static storage duration, and shall
not contain a reference to an identifier with internal linkage."

As standard does not require compiler to determine when it is safe to
reference an internal function in an external inline function, but instead
delegalizes such reference as a whole, gcc throws a warning for such code
under C99 compilation.  Warning produced by libc header is inhereted widely
and strips the option of using -Werror to track code sanity.

Replace static inline specifier with gnu89 extern inline. Latter "is used
only for inlining. In no case is the function compiled on its own", which
is slightly different from former semantically, but should produce the same
result here.

Change-Id: I6a3374498e5499d110e54468cf9d0d67d2debbe2