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]

Failed to set assignee to workingjubilee: invalid assignee

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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

[Amanieu]

This shouldn't be checking for unknown: that will just cause the tests to break when the disassembler is updated in the future. It's fine to leave them without instruction tests for now.
On second thought, that's probably fine, it will serve as a reminder to update the instruction tests.

[SpriteOvO]

Rebased.