Use jemalloc in dmd

[dlangBugzillaToGithub]

Andrei Alexandrescu (@andralex) reported this on 2015-01-26T21:46:39Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=14054

Description

There are a number of options, ranging from specifying jemalloc in the linking command line of the makefile on an opt-in basis, to distributing jemalloc's source itself.

jemalloc improves dmd's run time by 15%. It's licensed via a BSD 4-clause license which specifically eliminates the dreaded binary attribution clause.

[thewilsonator]

We have -lowmem now for where memory pressure is the major cause of slowdown and with a new GC on the way this is less necessary.