CMake: Support the LLVM_INSTALL_TOOLCHAIN_ONLY flag

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@189157 91177308-0d34-0410-b5e6-96231b3b80d8
Separates /fixed (no .reloc section) and /dynamicbase (enable ASLR)
in order to match link.exe's behaviour.

Patch by Ron Ofir.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@189159 91177308-0d34-0410-b5e6-96231b3b80d8