[Flang] Search flang_rt in clang_rt path (#151954)

The clang/flang driver has two separate systems for find the location of
clang_rt (simplified):

* `getCompilerRTPath()`, e.g. `../lib/clang/22/lib/windows`,
   used when `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=0`
* `getRuntimePath()`, e.g. `../lib/clang/22/lib/x86_64-pc-windows-msvc`,
   used when `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=1`

To simplify the search path, Flang-RT normally assumes only
`getRuntimePath()`, i.e. ignoring `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR`
and always using the `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=1` mechanism.
There is an exception for Apple Darwin triples where `getRuntimePath()`
returns nothing. The flang-rt/compiler-rt CMake code for library
location also ignores `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` but uses the
`LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=0` path instead. Since only
`getRuntimePath()` is automatically added to the linker command line,
this patch explicitly adds `getCompilerRTPath()` to the path when
linking flang_rt.

Fixes #151031

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