[mlir][Target] Improve ROCDL gpu serialization API (#95456)

This patch improves the ROCDL gpu serialization API by:
- Introducing the enum `AMDGCNLibraries` for specifying the AMD GCN
device code libraries to use during linking.
- Removing `getCommonBitcodeLibs` in favor of `AMDGCNLibraries`.
Previously `getCommonBitcodeLibs` would try to load all AMD GCN bitcode
librariesm now it will only load the requested libraries.
- Exposing the `compileToBinary` method and making it virtual, allowing
downstream users to re-use this method.
- Exposing `moduleToObjectImpl`, this method provides a prototype flow
for compiling to binary, allowing downstream users to re-use this
method.
- It also avoids constructing the control variables if no device
libraries are being used.

This patch also changes the behavior of the CMake flag
`DEFAULT_ROCM_PATH`. Before it would fall back to a default value of
`/opt/rocm` if not specified. However, that default value causes fragile
builds in environments with ROCm. Now, the flag falls back to the empty
string, making it clear that **the user must provide a value at LLVM
build time**.