RowHammer: Move inline asm rowhammer loop to assembly.

The code fragment presented in the RowHammer paper has a core loop of
LOAD row1/LOAD row2/CLFLUSH row1/CLFLUSH row2/MFENCE.

In the memtest86+ test environment, with each statement as a separate
expression, there were other extraneous memory references in the core
loop that may be confounding the memory access pattern. Remove them by
using a raw .S file.