mbed-ce@master + fixes + gcc 11 support (#135) ...

* 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

Fix some build errors

Unit tests: Remove redundant CMake target mbed-headers-base ...

Individual libraries' `target_h` stub headers have now all been moved
from `mbed-headers-base` to `mbed-headers-<library>`.

Note: Even though headers previously in `target_h` are technically
stubs/fakes too, they are used by not only unit tests but also regular
libraries when compiled for unit tests, because no target-specific HAL
implementation exists in this case. In order for regular library
sources to pick up `target_h` headers, those headers must

* have the same names as regular headers
* appear first in include paths

This is why those headers are part of `mbed-headers-<library>` and not
`mbed-stubs-<library>`. Before this refactoring, `mbed-headers-base`
was the first in unit tests' include paths.

Unit tests: Move drivers target_h stubs

CMake: unit-tests: Make drivers stub only depend on headers it uses ...

Previously the drivers stub library depended on `mbed-headers`, which
is a collection of all available headers in mbed-os. To make it easier
to separate the library, only depend on the headers we're using.

CMake: unit-tests: Move mbed-headers-drivers to the drivers directory ...

Move the header-only mbed-headers-drivers library the unit test stubs
depend on into the drivers component directory. This makes the drivers
stubs more self-contained and improves the composition of the library.

CMake: Move drivers stubs to the mbed-os/drivers directory ...

Move the drivers stub library into the drivers component directory.
so we can avoid duplicating the mbed-os source tree in a central
UNITTESTS folder.