This PR allows automatically fetching the latest jemalloc from
its dev branch by defining the environment variable
`JEMALLOC_SYS_GIT_DEV_BRANCH`. This is useful when debugging
whether certain bugs are already fixed upstream, as well as
for detecting whether upstream changes break our tests.
This PR adds two build jobs that test the dev branch on the
tier-1 x86_64 linux and osx targets.
This is an attempt to fix the tests, but the tests
still fail due to a libstd issue. Once we figure the
proper page size to use out we can set it in
jemalloc-sys' build.rs.
gnzlbg