* Fix rtos::Mutex stub, add trylock() method

* LowPowerTicker - Add ::attach(...) method + call callback

* mbed_power_management - rename sleep --> mbed_sleep to avoid conflicts

Fixes conflicts w/ unistd.h and boost::ut

* GCC 11/C++ 20 - Remove deprecated type_traits

* GCC 11/C++ 20 - Remove redundant templating of methods

* Warning - pragma ignore invalid-noreturn for unit tests

* Fix astyle errors

* Fix missing mbed_sleep() calls

This will make build failure if ticker optim defines are not present

(1000000 * LP/US_TICKER_PERIOD_DEN) > U32

They're now potentially useful, so document them as per the US_TICKER
defines.