[PATCH] RISC-V: Adjust stdint.h to stdint-gcc.h for rvv tests

[Robin Dapp]

> Huh, including stdint-gcc.h looks completely wrong.  What's the issue you are
> trying to solve?

The way I understood it is that that's a temporary workaround until
all multilib et al. (+testsuite) configurations are in place but I
haven't checked the details myself.  Eventually this should be done
properly so we can include the regular headers.  Kito might want to
comment as he dealt with it before.

I used #include <stdlib.h> for all those tests and Andreas Schwab reported:
 
  /usr/include/gnu/stubs.h:11:11: fatal error: gnu/stubs-ilp32d.h: No such file or directory

Regards
 Robin