compiler: handle abstract type in builtin numeric const value

Builtin_call_expression::do_numeric_constant_value can be called by
Array_type::verify_length before the determine types pass, so accept
an abstract type.

Test case is https://golang.org/cl/147537.

Fixes golang/go#28601

Change-Id: If4c3365af0f91ebaac0484666d6b318dcb748eb5
Reviewed-on: https://go-review.googlesource.com/c/147442
Reviewed-by: Cherry Zhang <cherryyz@google.com>