Intrinsic function naming scheme for P extension.

[kito-cheng]

I've create an issue on riscv-c-api-doc: riscv-non-isa/riscv-c-api-doc#18, which listed all current naming scheme for all intrinsic functions.

I think it would be great if we have consistent intrinsic function naming scheme for all extension.

[marcfedorow]

+1.
Also I disagree with __rv__sadd64 and __rv__ssub64 names.
There are __rv__kadd64 and __rv__ukadd64.
There are __rv__radd64 and __rv__uradd64.
I think we could similarly have __rv__add64 for signed tydes and __rv__uadd64 for unsigned.
(The same for subtraction insns).

[chuanhua]

Release v0.9.6 have changed __rv__ to __rv_.