[Offload] Fix the offload cache file triggering libc++ / libstdc++ mixing (#126313)

Summary:
We originally wanted `-stdlib=libc++` by default so that it could use
offloading support in libc++, however this causes issues with out the
Offloading proejct itself is built. Is the user builds the LLVM libs
with libstdc++ then uses this cache it will enable this option by
default for the ensuing build of the offloading libraries with the newly
build clang. This will cause a lot of linker failured because the C++
library doesn't match.

Long term I think the proper solution to this is to make better use of
clang configuration files, but I don't know a good way to do that by
default. For now just make it build right.

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