riscv: Enable clang

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

clang: Fix errors on zero kernel devices

"const TYPE *ptr" creates a non-const pointer to const data; for the
pointer itself to be rodata requires "TYPE *const ptr".

Taking the size of the pointer is caught by -Werror by default; suppress
that.

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

riscv/clang: Use integrated as

When not using integrated as, clang generates %got_pcrel_hi relocs which
are not supported by any currently released version of binutils.

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

Add workaround for llvm 11 inline asm regression

Without this an -O0 aarch64 build causes an assertion failure when
compiling inline asm in sel4runtime. See the linked phabricator
comments for analysis.

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

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.

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.

Set the cross compiler prefix to the triple

By default, clang looks for tools prefixed with the target triple.
Explicitly setting the target triple in the toolchain file allows seL4
tools to do the same.

Error if clang is used with riscv

Riscv is not yet supported with clang so error in
the llvm toolchain file and exit early.

Add ccache to the clang builds

Ccache increases compilation speeds dramatically
Add this to the clang toolchain

Do not use clangs builtin assembler

The builtin assembler in clang is not intended for
hand written assembly. Only that generated by clang.
Disable this and use the default one

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)

Add initial LLVM toolchain support