[clang] [llvm] [RISCV] Add riscv_packed.h for P extension intrinsics (PR #181115)

[via cfe-commits]

sihuan wrote:

Added `cross-project-tests/intrinsic-header-tests/riscv_packed.c`. This makes it easy to check which instruction each intrinsic actually lowers to.
A few spec divergences are flagged with TODO comments.

https://github.com/llvm/llvm-project/pull/181115