Don't prefix zero with 0x in assym.s.

The arm64 binutils only accepts 0 as an offset to the Load-Acquire Register
instructions where llvm will acceps both 0 and 0x0. The thread switching
code uses these with SCHED_ULE to block waiting for a lock to be released.
As the offset of the data to be loaded is zero this is safe, however it is
useful to keep the offset in the instruction to document what is being
loaded.

To work around this issue in binutils only generate the 0x prefix for
non-zero values.

Reported by: kan
Sponsored by: DARPA, AFRL