Fix asm constraints in __rmem_thread_start

The "fork" pseudo-instruction writes back to x0, so ensure that GCC
doesn't rely on it holding its previous value across the asm. This
fixes "illegal fetch address exception" reports.

Reported-by: Shaked Flur <shaked.flur@cl.cam.ac.uk>
Signed-off-by: Will Deacon <will.deacon@arm.com>