Impossible to override syscalls

[pavlohamov]

Can't override System V syscalls by my own implementation

1. Configure project: CONFIG_NEWLIB_LIBC=y
2. add trace into:zephyr/lib/libc/newlib/libc-hooks.c

 int _open(const char *name, int mode)
 {
+       printk(" %s '%s' %d\n", __FUNCTION__, name, mode);

4. add into main()
   FILE *f = fopen("file", "r+");
5. run it.
   So far so good
6. try implement your own "open"
7. lib/libc/newlib/libc-hooks.c:190: multiple definition of `_open';

Expected behavior
libc-hooks.c expected to have weak aliases

Impact
All syscall users

Environment (please complete the following information):

• Ubuntu 18 x64
• Toolchain: Zephyr
• Commit SHA f95a7fa