presets: Add core type

We require CMAKE_SYSTEM_PROCESSOR to be set so CMake can discover our
platform files. The platform files set architecture and ABI specific
options per language and toolchain. The ARMClang toolchain support in
CMake actually always required this variable: it was used to apply the
`--cpu` option for the ARMClang compiler.

Previously we set CMAKE_SYSTEM_PROCESSOR somewhere in MCU-Driver-HAL's
toolchain files. Now our default toolchain files don't set this variable
for simplicity and so they can remain generic for all cortex-m variants.
Users may set CMAKE_SYSTEM_PROCESSOR in a custom toolchain file for
their own convenience.

We set CMAKE_SYSTEM_PROCESSOR with our supported core types in our CMake
presets. This is for our own convenience when testing our current HAL
implementations.

Signed-off-by: Robert Walton <rob.walton@arm.com>