CMake: fix missing `SWIFT_CONCURRENCY_GLOBAL_EXECUTOR` value
`SWIFT_CONCURRENCY_GLOBAL_EXECUTOR` is defined in `stdlib/cmake/modules/StdlibOptions.cmake`, which is not included during the first pass of evaluation of the root `CMakeLists.txt`. It is available on subsequent evaluations after the value is stored in CMake cache. This led to subtle bugs, where `usr/lib/swift_static/linux/static-stdlib-args.lnk` didn't contain certain flags on clean toolchain builds, but did contain them in incremental builds.
Not having these autolinking flags in toolchain builds leads to errors when statically linking executables on Linux.
