[clang][test][RISCV] Introduce separate RUN lines for __risv_i test in riscv-target-features.c

Craig Topper pointed out in post-commit review of d05e1e99 that the
series of CHECK-NOT lines followed by CHECK would not work as intended
if __riscv_i came first in the input and was followed by the disallowed
lines.

The simplest solution is to just introduced separate RUN lines for the
__riscv_i check.