MLECO-6072: Adding ATfE/LLVM support

Adding support for newer LLVM based Arm Toolchain for
Embedded (ATfE). This is experimental at this stage but
has been tested with versions 19.1.5, 20.0.0 and 20.1.0.
For platforms, these changes have been tested on FPGA
implementations AN552 and AN555 and all variants for FVPs
supported.

Major additions are the CMake toolchain file and linker
scripts, along with required plumbing. CMake presets and
documentation have been updated too. Linker scripts now
have toolchain ID and extensions as suffix instead of just
the extension.

With the associated CMake toolchain file changes, support
for semihosting has also been refreshed, making it possible
to build and run semihosted applications on FVPs easily for
all toolchains. To enable easy boot verification, a simple
HAL test has been added. This needs to be built explicitly
and will produce a small application that boots the target
platform and calls HAL initialisation function.

Change-Id: Id114ca45ed53411c2647a2a0f775ba76609068ac
Signed-off-by: Kshitij Sisodia <kshitij.sisodia@arm.com>