[LLVM] Fix runtimes builds not triggering without LLVM_ENABLE_RUNTIMES (#94225)

Summary:
The runtimes builds create separate projects. Normally these use
`LLVM_EANBLE_RUNTIMES`. However, we can also use `LLVM_RUNTIME_TARGETS`
and `RUNTIMES_<target>_LLVM_ENABLE_RUNTIMES` to enable them. Currently,
if `LLVM_RUNTIME_TARGETS` isn't specified this is completely ignored.
This patch fixes the behavior so we can only specify the cross-compiling
versions.