Now that we properly only link in jemalloc when building executables, we have
far less to worry about in terms of polluting the global namespace with the
`free` and `malloc` symbols on Linux. This commit will primarily allow LLVM to
use jemalloc so the compiler will only be using one allocator overall.
Locally this took compile time for libsyntax from 95 seconds to 89 (a 6%
improvement).
e3b414d861