uwrap: Rewrite uwrap_libc_fns struct to pass strict aliasing rules.

Now uwrap is compiled with -O2 -fstrict-aliasing -Wstrict-aliasing=2
C compiler flags.

Also rename struct uwrap_libc_fns fns to uwrap_libc_symbols and
uwrap_load_lib_function to uwrap_bind_symbol (same for _uwrap_load_...
variant.

Signed-off-by: Robin Hack <hack.robin@gmail.com>
uwrap: Add pthread_exit() cleanup.

Threads should call pthread_exit() in the end of existence.

tests: Add pthread_exit() calls to test_glibc_thread_support.c.

Signed-off-by: Robin Hack <hack.robin@gmail.com>