Add P extension

[2011eric]

This commit add encodings for P Extension:
https://www.jhauser.us/RISCV/ext-P/RVP-instrEncodings-015.pdf

[chihminchao]

I can generate the encoding.out.h successfully. But there is a opcode question for rv32 and rv64. There are five instructions having same encoding but differenct name for mnemoic. There are
rv32 rv64
sslai vs psslai.w
ssha vs pssha.ws
sshar vs psshar.ws
sadd vs psadd.w
aadd vs paadd.w

Should't they be merged into one ? The decoding in implementation usally can find the first match pattern.

[2011eric]

@chihminchao
Thanks for reviewing.
For now I mark them as overlapping extensions in constraint.py.
And I do think it is necessary to include them in this repo due to better alignment with the spec.
I think I can mark them as pseudo_op to prevent the emission of their masks, or I can simply comment them out.

[aswaterman]

Thanks for submitting this PR. I have heard that there will be another round of renaming these instructions before the spec is submitted for architecture review. I think we should wait until that occurs, and leave this PR open in the meantime.