Remove all source files exclusive to Arm Compiler and IAR (#159) ...

* Remove all source files exclusive to Arm Compiler and IAR

* Also remove 'TOOLCHAIN_ARM'

* Remove targets.json entries for ARM and IAR

add separate cxx options for gcc (#80) ...

* add separate cxx options for gcc

* remove warning write-strings

Oops, missed a file

CMake: Fix escaping of quotes in response file ...

We put macros in a response file compile_time_defs.txt and pass it
to the compiler. Adding a pair of single quotes around each -D flag
ensures macro values are quoted correctly.

For example,
* String
  * target_compile_definitions(): either FOO="BAR" or FOO=\"BAR\"
  * response file: '-DFOO="BAR"'
  * actual definition: #define FOO "BAR"
* Array of integers
  * target_compile_definitions(): FOO={1, 2, 3}
  * response file: '-DFOO={1, 2, 3}'
  * actual definition: #define FOO {1, 2, 3}

CMake: Enable improved armclang support in CMake 3.21 ...

CMake versions 3.20 and below always add compiler and linker flags
based on `CMAKE_SYSTEM_PROCESSOR`, for example `--cpu=Cortex-M33`.
This sometimes overrides flags we set in tools/cmake/cores/*.cmake
and results in link failure or unbootable binaries. To workaround
this, we added more linker flags to "counter" what CMake automatically
adds.

From CMake 3.21 onwards, CMake by default does not add flags to
armclang anymore, and it fully relies on projects to set all flags.
In this case we do not need to set `CMAKE_SYSTEM_PROCESSOR` or use
workarounds anymore. We still turn on workarounds when an older
version of CMake (3.19 and 3.20) is used, but in the future we might
require users to have at least CMake 3.21.

Make changes for Cortex-A5 support

Unify the cores/Cortex-A.cmake

CMake: add mbed_ prefix to our CMake files ...

To avoid conflicts as we expose our CMake list files via CMAKE_MODULE_PATH. Some files already include it, this
aligns the rest of files.

I leave app.cmake as it is - it's user facing and it would be breaking change. We can clean this one for the next major version.