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

Remove ARMC5 reference from Mbed OS platform

Add mstd::is_constant_evaluated ...

GCC 9 and sufficiently-new Clang (including ARM Compiler 6.13) give us
access to the C++20 (draft) `is_constant_evaluated` test. Allows us to
restrict code to compile-time only.

This is particularly useful when using assembler intrinsics, which the
compiler cannot optimise or compile-time evaluate. It allows us to write
something like

    constexpr uint32_t reverse_bits(uint32_t x)
    {
        if (is_constant_evaluated(x)) {
            // C code to do calculation here
            return x;
        } else {
            // an assembler intrinsic that cannot be optimised
            return __RBIT(x);
        }
    }

This can then be totally compile-time given a constant input.

(In this example, ultimately it would be a good idea to include this
functionality in CMSIS's GCC `__RBIT`, which needs it because GCC
requires use of assembler. Clang implements `__RBIT` as a built-in,
meaning it can already compute it at compile-time, so does not benefit.).

Split mbed_cxxsupport.h up - add namespace mstd ...

Regularise the C++ support glue, adding `<mstd_type_traits>` etc.

These include the base toolchain file, backfill `namespace std` as much
as possible for ARM C 5, and then arrange to create unified
`namespace mstd`, which can incorporate toolchain bypasses
(eg `mstd::swap` for ARM C 5, or `mstd::atomic`), and include local
implementations of post-ARM C++14 stuff.

All APIs in `namespace mstd` are intended to function as their
`namespace std` equivalent, and their presence there indicates they
are functional on all toolchains, and should be safe to use in
an Mbed OS build (including not unreasonable memory footprint).

See README.md for more info.