trivial: Style check_arch_compiler.cmake

Signed-off-by: Stefan O'Rear <sorear@fastmail.com>

riscv: Use a multiarch triple for clang

Much like x86_64, the riscv64 GNU toolchain handles both riscv32 and
riscv64 depending on compile flags.

Signed-off-by: Stefan O'Rear <sorear@fastmail.com>

convert license headers to SPDX

Includes license review of 3rd-party files, adding missing upstream
headers, and moving the UNSW OZPLB license into its own file rather
than replicating it in the header.

Use a clang specific compiler check

The clang compiler check has to be different to the gcc.
Instead of checking if platform specific intrinsics are defined,
check the provided triple (i.e the target clang thinks it is
building for)

cmake: use check_arch_compiler via function

CMake modules that declare a function are more consistent than modules
that have an effect just from importing them.

check_arch_compiler.cmake: aarch32 ArmV8 is valid

When building aarch32 ArmV8, __ARM_ARCH_8A__ is set and not
__ARM_ARCH_7A__. Additionally, __ARM_ARCH_7A__ is set for ArmV7ve, not
__ARM_ARCH_7VE__

CMake: Add check_arch_compiler.cmake

This triggers errors if the incorrect compiler is selected.