[Darwin] Respect -fno-unroll-loops during LTO.

Currently -fno-unroll-loops is ignored when doing LTO on Darwin. This
patch adds a new -lto-no-unroll-loops option to the LTO code generator
and forwards it to the linker if -fno-unroll-loops is passed.

Reviewers: thegameg, steven_wu

Reviewed By: thegameg

Differential Revision: https://reviews.llvm.org/D76916