libc: implement stub function for execve

Porting CRuby now, and this is call execve and etc.  I won't support exec from
Ruby, so this function just need to fail.

Signed-off-by: KANATSU Minoru <icc.pot.tyew272@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

libc/process/execve.cc

+#include <unistd.h>
+#include <osv/stubbing.hh>
+
+int execve(const char *path, char *const argv[], char *const envp[])
+{
+    WARN_STUBBED();
+    return libc_error(ENOEXEC);
+}