Clarify status of varargs support of foreign import ccall
It's currently a bit unclear whether foreign functions expecting varargs can be imported with
foreign import ccall. Currently there is one example of such a usage in
T5423 which ends up breaking under Darwin's AArch64 calling convention (see https://developer.apple.com/documentation/xcode/writing_arm64_code_for_apple_platforms#//apple_ref/doc/uid/TP40013702-SW1 and !3641 ).
Given that there seems to be recognition among users that there this should be expected to work I think we should formally declare varargs as unsupported. Users who need to call functions with varargs can instead use
foreign import capi.