T-Head has a range of vendor-specific instructions.
Therefore it makes sense to group them into smaller chunks
in form of vendor extensions.
This patch adds the XTheadCmo extension, a collection of T-Head specific
cache management operations.
The 'th' prefix and the "XTheadCmo" extension are documented in a PR
for the RISC-V toolchain conventions ([1]).
In total XTheadCmo introduces the following 21 instructions:
Contrary to Zicbom, the XTheadCmo instructions don't have a constant
displacement, therefore we have a different syntax for the arguments.
To clarify this is intended behaviour, there is a set of negative test
for Zicbom-style arguments in x-thead-cmo-fail.s.