Intrinsic naming

[marcfedorow]

The use of two underscores in identifiers is reserved
for the compiler's internal use according to the ANSI-C standard.
Intrinsics are indeed "the compiler's internal" stuff.
This means that B-ext and K-ext scheme are most likely improper.
Still two underscores after __rv seem to be redundant.
__rv_insn and __rv_v_insn could probably be the best decision.
See also: #44, riscv-non-isa/riscv-c-api-doc#18

[chuanhua]

Changed "__rv__" prefix to "__rv_" in v0.9.6 release (0eeb572)