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

Add SingletonPtr.cpp to unittests source

Fix some build errors

Clean up top-level build file and build options, fix build issues on MinGW

Allow unit tests to be compiled with -fno-exception

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 randLIB.h ...

The stub header randLIB.h overrides the header of the same name in
platform/randlib/ which is an external repository vendored into
the mbed-os codebase. As the repository is synchronized regularly,
it is better not to put overrides there, so we put the randLIB.h
stub into the regular platform doubles directory.

Unit tests: Move platform target_h stubs ...

Stubs previously in UNITTESTS/target_h/ have the same names as
regular Mbed OS headers, intending to override the latter directly.
We move platform target_h stubs into
platform/tests/UNITTESTS/doubles/platform/.

Note: nvic_wrapper.h is normally implemented and used by Mbed targets
as needed. But as unit tests do not have a real target, we treat it
as a stub for the platform.

Unit tests: Move cmsis target_h stubs ...

This creates a new mbed-headers-cmsis which is always and only used
by mbed-headers-platform, so we add the former to the laters' linked
targets.

test: Make MBED_ASSERT failures fail ...

Make MBED_ASSERT failures fail the unit test case. Without this, we
might not notice the assertion failure as it wouldn't be bubbled up to
googletest.

CMake: unit-tests: Remove --coverage option from mbed-stubs-platform ...

We set the --coverage flag globally in UNITTESTS/CMakeLists.txt. We
shouldn't need to also set it on mbed-stubs-platform, so remove it.

CMake: unit-tests: Split platform headers out of mbed-stubs-headers ...

We have a central collection of "stub headers", which makes reasoning
about dependencies rather difficult, as it forces every stub library to
depend on all available stub headers. The standard approach would be for
each stub library to expose its public headers, and its dependents to
explicitly specify a dependency on the stub library containing the
headers it needs. This is a more modular design than creating a
header-only monolith library. Move the platform stub headers from this
central library into the mbed-stubs-platform library to increase
modularity.

mbed-stubs-connectivity now depends on the mbed-stubs-platform because
it requires some headers which were moved to mbed-stubs-platform.

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

Previously the platform 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 actually using.

CMake: unit-tests: Move mbed-headers-platform to platform directory ...

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

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

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