[llvm] [RISCV] Support P extension ABSW instruction. (PR #165047)

[Piotr Fusik via llvm-commits]

pfusik wrote:

Where's the spec of the P extension?
I don't see it at https://github.com/riscv/riscv-isa-manual and at riscv.org I found https://riscv.atlassian.net/browse/RVS-1135 which links to https://github.com/riscv/riscv-p-spec which contains `KABSW` not `ABSW`.

https://github.com/llvm/llvm-project/pull/165047