Add a LLVM_USE_LINKER that defines the linker to use when building LLVM

Summary:
This string parameter is passed to -fuse-ld when linking. It can be
an absolute path to your custom linker, otherwise clang will look for
`ld.{name}`.

Reviewers: davide, tejohnson, pcc

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D28738

llvm-svn: 292047