[CMake] Support Macros in Linux
For compiling codes required for macro support, we now need swiftc
compiler in the build machine.
Unlike Darwin OSes, where swiftCore runtime is guaranteed to be present
in /usr/lib, Linux doesn't have ABI stability and the stdlib of the
build machine is not at the specific location. So the built compiler
cannot relies on the shared object in the toolchain.
(cherry picked from commit 9c9010e5b7506c4bd89c5ef0148e4741b32a1a2c)
(cherry picked from commit 040d15d7e8727a192dc3944199bfc93257bef78c)
