[FLANG] Fix MSVC + clang-cl build

Flang build on windows with MSVC environment and clang-cl compiler
requires clang_rt.builtin.${target} library. This patch allows us to
locate and include this link library. This is mostly needed for flang
runtime and associated unittests as it requires the uint128 division
builtin function __udivti3.

Reviewed By: DavidTruby

Differential Revision: https://reviews.llvm.org/D138023