Run valgrind with fair scheduling when available

[brson]

Will find more multithreading problems.

http://valgrind.org/docs/manual/manual-core.html#manual-core.pthreads_perf_sched

[catamorphism]

Nominating for maturity milestone 4, "well-covered".

[graydon]

accepted for well-covered milestone

[emberian]

triage bump

[pnkfelix]

visiting for triage, email from 2013-09-02. Nothing to add.

[sanxiyn]

--fair-sched is new in Valgrind 3.8.0, but Rust now requires Valgrind 3.8.0 for jemalloc anyway (#9237). It also needs platform support, but while --fair-sched=yes will terminate with an error when there is no platform support, --fair-sched=try will fallback. I think we can try this now.

[pnkfelix]

assigning P-low.

[reem]

Triage: Given sanxiyn's comment, could implement, but not sure where a fix for this goes.