dynamically add hooks from libc

The hook table should contain the pthread functions
that are safe to call without wrapping, or
the wrapped functions.
This means there's no need for:
    {"libc_function_name", libc_function_name},

It also makes it easier to split hooks.c into separate files.

Signed-off-by: Adrian Negreanu <adrian.m.negreanu@intel.com>