Revert "Add hidden visibility to internal function prototypes".

The commit

commit 568ff429
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Mon Aug 21 05:50:38 2017 -0700

    Add hidden visibility to internal function prototypes

breaks the build of the testsuite for many platforms:

https://sourceware.org/ml/libc-testresults/2017-q3/msg00300.html

The errors are of the following form, building math/atest-exp:

[...]
/scratch/jmyers/glibc-bot/build/glibcs/aarch64-linux-gnu/glibc/stdlib/rshift.o: In function `__mpn_rshift':
/scratch/jmyers/glibc-bot/build/glibcs/aarch64-linux-gnu/glibc-src/stdlib/rshift.c:45: undefined reference to `__assert_fail'
/scratch/jmyers/glibc-bot/install/compilers/aarch64-linux-gnu/lib/gcc/aarch64-glibc-linux-gnu/7.2.1/../../../../aarch64-glibc-linux-gnu/bin/ld: /scratch/jmyers/glibc-bot/build/glibcs/aarch64-linux-gnu/glibc/math/atest-exp: hidden symbol `__assert_fail' isn't defined
/scratch/jmyers/glibc-bot/install/compilers/aarch64-linux-gnu/lib/gcc/aarch64-glibc-linux-gnu/7.2.1/../../../../aarch64-glibc-linux-gnu/bin/ld: final link failed: Bad value

This test is using various objects from the build of libc.  Some of
those objects contain references to __assert_fail.  Because those
references are hidden, they cannot refer to __assert_fail from
libc.so.  Given the tests using internal objects those symbols in
libc.a can't safely be made hidden, so this patch reverts the problem
commit until any alternative approach that doesn't break the build can
be found.