Stop storing -1 to the clone flags

This is kind of scary: I was poking around glibc to find out why the
bump didn't build and I stumbled across an upstream bugfix that needs to
be applied to the RISC-V assembly code.

This matches the upstream commit
0cb313f7.  The MIPS patch there pretty
much applies to the RISC-V stuff, so I just did it manually.  I've
tested to see that this builds, but nothing else.