use the default jemalloc prefix

By default, jemalloc uses a prefix on OS X, iOS and Windows where
attempting to replace the platform allocator is broken. Avoiding a
prefix allows jemalloc to override the weak symbols from libc. This
eliminates the fragmentation and performance hit caused by having two
general purpose allocators in the same process.

The time spent on LLVM passes in the Rust compiler on Linux is cut by
10% and peak memory usage is reduced by 15%.