Enable -threads by default.
Enable -threads by default.

LLD supports multi-threading, and it seems to be working well as
you can see in r287140. In short, LLD runs a few percent to 30%
faster with -threads and more than 50% faster if you are using
-build-id (your mileage may vary depending on your computer).

However, I don't think most users even don't know about that because
-threads is not a default option.

This patch enables it by default.

Discussion thread:
http://lists.llvm.org/pipermail/llvm-dev/2016-November/107160.html

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@287237 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld