[Flang] LLVM_ENABLE_RUNTIMES=flang-rt (#110217)
Extract Flang's runtime library to use the LLVM_ENABLE_RUNTIME
mechanism. It will only become active when
`LLVM_ENABLE_RUNTIMES=flang-rt` is used, which also changes the
`FLANG_INCLUDE_RUNTIME` to `OFF` so the old runtime build rules do not
conflict. This also means that unless `LLVM_ENABLE_RUNTIMES=flang-rt` is
passed, nothing changes with the current build process.
Motivation:
* Consistency with LLVM's other runtime libraries (compiler-rt, libc,
libcxx, openmp offload, ...)
* Allows compiling the runtime for multiple targets at once using the
LLVM_RUNTIME_TARGETS configuration options
* Installs the runtime into the compiler's per-target resource directory
so it can be automatically found even when cross-compiling
Also see RFC discussion at
https://discourse.llvm.org/t/rfc-use-llvm-enable-runtimes-for-flangs-runtime/80826
