Comply with RISC-V toolchain conventions for vendor extensions.

[resistor]

Cheriot v1 needs to conform to standardized conventions for RISC-V vendor extensions in order to be acceptable to upstream LLVM. These conventions are documented here: https://github.com/riscv-non-isa/riscv-toolchain-conventions/blob/main/src/toolchain-conventions.adoc#conventions-for-vendor-extensions

• Xcheriotv1 subtarget feature is defined and used
• Instruction mnemonics are prefixed with "ct."
• CSRs are prefixed with "ct."
• Relocations use R_RISCV_VENDOR to exist in a vendor-specific namespace
• All prefixes and identifiers are upstreamed into the official toolchain conventions document

[davidchisnall]

We will also need to provide assembly macros or, ideally, aliases (so inline assembly works) for the current instruction mnemonics.

[resistor]

We will also need to provide assembly macros or, ideally, aliases (so inline assembly works) for the current instruction mnemonics.

Won't that run into an issue with name collisions with Zcheri?

[davidchisnall]

No Zcheri is renaming all of the instructions to have stupid names (e.g. candperm -> acperm). I think hiding the aliases behind an opt-in flag should be fine.