[clang] [Clang][AArch64] Define __USER_LABEL_PREFIX__ to # for ARM64EC (PR #78913)

[Billy Laws via cfe-commits]

bylaws wrote:

@efriedma-quic Fair enough, I found this cleaned up things on the compiler-rt side but even then it still needed changes to handle concatenating a #, and I'd imagine that to be the same for any other existing user. It would still be nice to have something equivalent to A64NAME for user code but that probably better fits in mingw.

https://github.com/llvm/llvm-project/pull/78913