libc: Allow more complex expressions for CALL_BLOCK first argument

For the case where the compiler supports blocks we only allow the first
expression to have an operator if it has as high precedence as a
function call, which for blocks effectively means member access and
subscripting only, not even a dereference nor a cast. Parenthesise this,
as is the case for the missing compiler support case, so that it's more
general.