rustc_target: RISC-V: feature addition batch 2

This commit adds unprivileged ratified extensions that are either
dicoverable from the `riscv_hwprobe` syscall of the Linux kernel (as of
version 6.14) plus 1 minus 3 extensions.

Plus 1:

*   "B"
    This is a combination of "Zba", "Zbb" and "Zbs".
    Note:
    Although not required by the RISC-V specification, it is convenient to
    imply "B" from its three members (will be implemented in LLVM 21/22) but
    this is not yet implemented in Rust due to current implication handling.
    It still implies three members *from* "B".

Minus 2:

*   "Zcf" (target_arch = "riscv32" only)
    This is the compression instruction subset corresponding "F".
    This is implied from RV32 + "C" + "F" but this complex handling is
    not yet supported by Rust's feature handling.
*   "Zcd"
    This is the compression instruction subset corresponding "D".
    This is implied from "C" + "D" but this complex handling is
    not yet supported by Rust's feature handling.
*   "Supm"
    Unlike regular RISC-V extensions, "Supm" and "Sspm" extensions do not
    provide any specific architectural features / constraints but requires
    *some* mechanisms to control pointer masking for the current mode.
    For instance, reported existence of the "Supm" extension in Linux means
    that `prctl` system call to control pointer masking is available and
    there are alternative ways to detect the existence.

Notes:

*   Because this commit adds the "Zca" extension (an integer subset of the
    "C" extension), the "C" extension is modified to imply "Zca".

0 files changed, 0 insertions, 0 deletions