gcc.cmake Add additional RISCV toolchains

Search for more toolchains if the first one cannot be found.

Signed-off-by: Kent McLeod <Kent.Mcleod@data61.csiro.au>

gcc.cmake: Don't use CACHE when setting variables

The toolchain file is invoked in many different CACHE contexts and
shouldn't assume any behavior about the cache. Variables set in the file
will also be inherited by any scope that calls project() and so it is
not necessary to write any variables into the CACHE.

gcc.cmake: Detect if toolchain file is templated

If the toolchain file is not templated, then any template variables
won't be initialized correctly. Add a test for overwriting
CROSS_COMPILER_PREFIX only if the templated value is valid.

Set CROSS_COMPILER_PREFIX in gcc.cmake template

Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

Remove unnecessary variables in toolchain files

Setting `-target` flag explicitly is unnecessary. Whenever this needs to
be explicitly passed through, the coompiler id should be checked.

Changes to the musl build system makes the explicit `C_COMPILER`
variable useless.

Specify the C compiler in a variable for user lvl

Having the basic compiler name in a variable lets us pass it around to
any other user level project (e.g musllibc)

Fix style complaints in `gcc.cmake`

Unrelated to the rest of the changeset on this branch but a good
opportunity to make the code quality checks happy.

Add support for arm Xcompilers on Red Hat distros

This change introduces a `gcc` hunting helper function to `gcc.cmake` in
order to help us find an appropriately prefixed `gcc` for the target
being built for. We use this helper to find 32b ARM cross-compiling
`gcc`s for both Debian and Red Hat based distros.

gcc.cmake: Add option for coloured gcc output

Setting the environment variable GCC_COLORS or CMake option
FORCE_COLORED_OUTPUT will cause GCC's colour messages to be outputted.

gcc.cmake: Configure ccache in toolchain file

This should be a reasonable default option.

CMake: Possibly set toolchain file automatically

If CMAKE_TOOLCHAIN_FILE is unset in seL4Config.cmake then generate one
that selects the correct toolchain based on configured kernel arch
settings.

This means that initialising a build goes from:
`cmake -DCMAKE_TOOLCHAIN_FILE=gcc.cmake -DAARCH32=ON -G Ninja -C
../configs/ARM_verified.cmake ..`
to:
`cmake -G Ninja -C ../configs/ARM_verified.cmake ..`

gcc.cmake is modified to be used as an input to configure_file.
@KernelArch@ and other @@ arguments will be replaced with the
configuration settings. The file will still work if passed directly to
cmake via -DCMAKE_TOOLCHAIN_FILE=gcc.cmake without being templated.

cmake: Update the RISCV build to work with GCC 8.2.0

CMake: Move APPLE host check into x86 branch

This is to more clearly record the OSX host + x86 target assumption
when setting CROSS_COMPILER_PREFIX to "x86_64-unknown-linux-gnu-"

Use Linux cross-compiler if building for x86_64 on OS X

Even if we are building for x86_64, we still need a cross compiler to be able to
build ELF binaries.

OS X cross compilers can be installed via brew from here:
https://github.com/SergioBenitez/homebrew-osxct

cmake: Add RISC-V default toolchains to gcc.cmake

cmake: Convience argument for arm hardware floating point toolchain

Rename ARM toolchain flag to aarch32

Add toolchain flag for aarch64

cmake: Easier setting of ARM cross compiler

Allows for an ARM cross compiler to be specified with just -DARM=TRUE when initializing
cmake, instead of having to specify a full cross compilation prefix. This provides no
additional functionality, but provides a nicer interface for users

Add a CMake based build system

This commit adds an alternate build system using CMake that operates indepenently of
the existing Kconfig+Kbuild+make based build system