riscv: NO_INLINE halt

This function is called at most once per boot, but clang wants to inline
it by default.

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

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.

riscv: don't translate 'halt'

Removes a spurious warning from the C parser about ignoring a previous
MODIFIES spec.

riscv: Correct stack setting for idle thread

This sets the idle thread to have a stack top, instead of a stack bottom, which means
it no longer needs to have a custom optimize pragma to avoid using the stack.

RISCV: Place TODOs in the source

[SELFOUR-1156] RISC-V Port

Experimental release that supports both RV32 and RV64