[llvm] Fix Xtensa Bazel build (PR #173073)

[NAKAMURA Takumi via llvm-commits]

https://github.com/chapuni commented:

@keith Xtensa shouldn't be in `DEFAULT_TARGETS`, since this hasn't been enabled in CMake side.

Instead, could we add it in our provided workspace? (I don't use it, though)

https://github.com/llvm/llvm-project/pull/173073