cmake: modules: Introduce FindThreads module

Some third-party modules uses 'find_package(Threads REQUIRED)' to check
if threads implementation is supported.

The original implementation tries to find threads library using various
methods (e.g. checking if pthread library is present or compiling
example program to check if the implementation is provided by libc), but
it's not able to detect pthread implementation provided by Zephyr.

Signed-off-by: Patryk Duda <patrykd@google.com>