We pass 'environ' to execve() in arch_avc2_exec(), so that we retain the
environment in the current process. But the declaration of 'environ' is
a bit weird - it doesn't seem to be in a standard header, requiring a
manual explicit declaration. But, we can avoid needing to reference it
explicitly by using execv() instead of execve(). This removes a clang
warning.
Signed-off-by: David Gibson