[AArch64] Fix generated types for ARM64EC variadic entry thunk targets (#80595)

ISel handles filling in x4/x5 when calling variadic functions as they
don't correspond to the 5th/6th X64 arguments but rather to the end of
the shadow space on the stack and the size in bytes of all stack
parameters (ignored and written as 0 for calls from entry thunks).

Will PR a follow up with ISel handling after this is merged.