gn build: Move target flags from toolchain to a .gni file.

While here, add a use_lld flag and default it to true when using
clang on non-mac.

Differential Revision: https://reviews.llvm.org/D56710

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351248 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed