Fix RISC-V Packed SIMD intrinsics compilation

[SpriteOvO]

Fixes rust-lang/rust#129593.

Currently the P extension is still unratified, so there is no support for it in the upstream LLVM for now, and thus no LLVM built-in functions or serialization of instructions are provided.

This PR implements quick fixes to some of the instructions to fix them causing compilation failures (see issue rust-lang/rust#129593). Calling it a "quick fix" because ideally we could replace all .insn with LLVM built-in functions in the future when LLVM supports the P extension.

This PR also adds assert_instr(unknown) to each function so that we can at least make sure they compile. Since there is no serialization yet, the stdout of assert_instr is like

---- core_arch::riscv_shared::p::assert_kabs16_unkno1wn stdout ----
disassembly for stdarch_test_shim_kabs16_unkno1wn: 
         0: <unknown>
         1: ret
thread 'core_arch::riscv_shared::p::assert_kabs16_unkno1wn' panicked at crates/stdarch-test/src/lib.rs:172:9:
failed to find instruction `unkno1wn` in the disassembly
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    core_arch::riscv_shared::p::assert_kabs16_unkno1wn

So I left "unknown" here, also with the idea that if LLVM upstream provides support for the P extension at some point in the future, we can know in time and then update our implementation.

r? @workingjubilee
CC @Amanieu

[rustbot]

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[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @workingjubilee (or someone else) some time within the next two weeks.

[SpriteOvO]

r? @Amanieu