Move store conditional result checking from SimpleAtomicCpu write

Move store conditional result checking from SimpleAtomicCpu write
function into Alpha ISA description.  write now just generically
returns a result value if the res pointer is non-null (which means
we can only provide a res pointer if we expect a valid result
value).

--HG--
extra : convert_revision : fb1c315515787f5fbbf7d1af7e428bdbfe8148b8