This patch disables the stdcxx-compat check for the sm-fuzzing build which
requires patching autospider as well. Furthermore, it switches the build
to linux64-clang-6-pre because the older clang 3.9 does not support trace-pc
instrumentation. Finally, it excludes fuzzing parts from the vanilla allocation
check.

MozReview-Commit-ID: FdhCIFdUore

--HG--
extra : rebase_source : c41bda01cb42f2ef0cd5a1675d88bdb55d9dc8c9