We will disable some optimizations, because the function
row_ins_clust_index_entry_low() would fail to compile
ever since commit a73eedbf
changed the definition of srw_mutex::wr_unlock() to use
fetch_sub() instead of fetch_and().

For some reason, applying this work-around does not fix the
"could not split insn" error for that commit,
while it does work for
commit 277ba134.