microblaze: fix variable name collision with syscall macros

If a function passes in a variable named "ret", the code will miscompile
when it declares a local ret variable.  In some cases, it's even a build
failure like so:
../sysdeps/unix/sysv/linux/spawni.c: In function '__spawni_child':
../sysdeps/unix/sysv/linux/spawni.c:289:5: error: address of register variable 'ret' requested
	while (write_not_cancel (p, &ret, sizeof ret) < 0)