Since __syscall_clock_gettime and __start_context are internal symbols
for Linux/x86-64, mark them with attribute_hidden.
[BZ #18822]
* sysdeps/unix/sysv/linux/x86_64/init-first.c
(__syscall_clock_gettime): Add attribute_hidden.
* sysdeps/unix/sysv/linux/x86_64/makecontext.c
(__start_context): Likewise.