fix microblaze syscall register clobbers

the kernel entry point for syscalls on microblaze nominally saves and
restores all registers, and testing on qemu always worked since qemu
behaves this way too. however, the real kernel treats r3:r4 as a
potential 64-bit return value from the syscall function, and copies
both over top of the saved registers before returning to userspace.
thus, we need to treat r4 as always-clobbered.