'c' and 'v' do not work as asm clobber or output registers

[rweather]

Specifying "c" or "v" as clobber registers give an error, rather than being supported as described here.

int main(void)
{
    unsigned char a = 0x86;
    asm (
        "rol"
        : "+a"(a)
        :: "c"
    );
    asm (
        "bit llvm-mos/llvm-mos-sdk#27"
        : "+a"(a)
        :: "v"
    );
    return 0;
}

Result:

$ mos-common-clang asm-flags.c 
asm-flags.c:7:12: error: unknown register name 'c' in asm
        :: "c"
           ^
asm-flags.c:12:12: error: unknown register name 'v' in asm
        :: "v"
           ^
2 errors generated.

Attempting to use "c" (or "v") as an output register compiles OK, but causes a link-time crash and stack trace.

int main(void)
{
    unsigned char a = 0x86;
    unsigned char c;
    asm (
        "rol"
        : "+a"(a), "=c"(c)
        ::
    );
    return 0;
}

Result:

LLVM ERROR: unable to translate instruction: call (in function: main)

crash.txt

[rweather]

clang version 17.0.0 (/~https://github.com/llvm-mos/llvm-mos 09aa314)
Target: mos-common
Thread model: posix
InstalledDir: /home/rweather/llvm-mos/bin
Configuration file: /home/rweather/llvm-mos/bin/mos-common.cfg