Prepend all library intrinsics with `#` when building for Arm64EC (#87542)

While attempting to build some Rust code, I was getting linker errors
due to missing functions that are implemented in `compiler-rt`. Turns
out that when `compiler-rt` is built for Arm64EC, all its function names
are mangled with the leading `#`.

This change removes the hard-coded list of library-implemented
intrinsics to mangle for Arm64EC, and instead assumes that they all must
be mangled.