[llvm-jitlink] Use concurrent linking by default.
Adds a -threads option to llvm-jitlink. By default llvm-jitlink will now use a
DynamicThreadPoolTaskDispatcher with the number of materialization threads set
to whatever is returned by std::hardware_concurrency(). This brings the default
in-place linking behavior in line with the concurrent linking that is used for
-oop-executor and -oop-executor-connect mode.
In-place linking on the main thread can be forced by passing -threads=0.
