[llvm] [win][arm64ec] More fixes for building and testing Arm64EC Windows (PR #151409)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

LLVM supports unnamed global variables and function; if such a function actually needs a name, we invent one later. (See Mangler::getNameWithPrefix.)  Not really that useful, or frequently used, but it's there.  I guess we can make AArch64Arm64ECCallLowering introduce names if they're necessary.  If you're not going to fix it, please open a bug.

Please split out the available_externally fix, with an appropriate testcase.

https://github.com/llvm/llvm-project/pull/151409